Right, no easy way to answer this... It varies, and varies, and not a lot of it's detailed anywhere. It depends on faith, and region, race... The specific church, et cetera.
Here's some relevant info from Ed on Candlekeep:
Hi, fourthmensch. Certainly I can explain. Sune and Sharess (and therefore, in almost all cases, their churches, too) wouldn't care about the married or unmarried state of persons indulging in lustful practises. Wherefore performing such short-term marriages wouldn't be clerical policy for them (except perhaps to indulge wanton faithful desiring the thrill of turning a marriage ceremony into debauchery).
However, the clergy of Siamorphe, representing a deity catering to the nobility, are concerned both with being popular/useful to nobles, AND safeguarding the rights, privileges, and powers of nobles. Bastard children, loss of virginity, and so-called "immoral" behaviour are matters far more important to noble families than to the general populace.
So legitimate but very-short-term marriages allow nobles to ah, enjoy other nobles (or commoners) whilst at the same time protecting the rights, status, and wealth of the participants and their families.
For example, a commoner female (or her family) can't easily later demand monies or other compensation from a noble male, in return for her lost maidenhead, when she willingly (the clergy will attest to this, for they test for it, with witnesses and spells) participated in a known-to-be-dusk-until-dawn marriage.
In like manner, a noble female can experience the joys of lusty male partners without said partners gaining any claim to becoming part of her family (so she can, er, taste dungsweepers instead of restricting herself to any chinless foolheaded fops who happen to be noble).
And so on. A father can promise his daughter in marriage to anyone who can slay the Dread Beast Barthos or recover the Lost Sacred Silver Apple without blindly binding his family to an evil, grasping lout or sacrificing her future and lifelong happiness -- because she can be shrouded in disease-averting, ironguard, and healing spells for a night and just put up with things for the one night. Elder nobles who don't want to share their fortunes or sacrifice their freedom can indulge in brief flings without sacrificing respectability (remember, in most places in the Realms no one deity is paramount [Lantan, with Gond, would be one of the exceptions], so the moral code of no one deity can dominate; whereas in our real world North America Christianity dominates and decides what's "respectable" or not, in the Realms all of the gods have rightful standing, so as ridiculous/insincere/overly convenient as a church-legitimized one-night stand might seem to us, it's not viewed that way by the majority of Faerunians...individuals who cleave to this god or that may sneer or deride or be scandalized, but would have a much harder time than many of our real-world fanatics do in gathering popular support for their personal views).
I must preface my answer to your second question with a gentle suggestion (please understand I'm not trying to be critical here) that your question to me, as phrased, betrays a North American Christian-dominated viewpoint. You speak of people "in serious relationships sowing some serious oats" as "lax mores."
Though I agree that in general the Realms does follow rather romanticized courtly love-plus-feminine-equality values (or at least, that's what I designed it to have), neither I nor the nigh-immortal [not everyone, notice, just the Chosen and other live-for-many-centuries folks, and nobles and royalty who consider themselves 'above' laws and social rules, or to be the people who set such laws and rules] fictional Realms characters I've created view "sleeping with" people as being incompatible with having deep, committed relationships with someone else. So they don't see it as "lax" at all (and by the way, neither did a LOT of real-world American people of a certain generation, during the 1960s/Woodstock generation -- making love to Person A was seen as having nothing at all to do with being life-bonded to Person B).
Yes, The Simbul and Elminster DO love each other. Deeply. Yet neither of them would define faithfulness to the other as having anything at all to do with sex. So, yes, "swinging" between committed or married couples isn't seen as Bad by a lot of Faerunians, in many places and situations (though among most citizens across the Realms, it would be).
[It's fashionable among some noble classes and a LOT of "wannabe noble" rich, rising merchants, and frowned upon in places with small, stable populations where warfare or monster predations haven't forced folk into desperate survival measures (telescoping survivors down into a single extended family of multiple husbands and wives, for example).]
I make no apology for this mental separation between love and lust. Outliving lover after lover, family after family, (many of) your own children, realm after realm, and so on will do that to you. You grab physical love when you can, and search for long-term partners with a desperate hunger.
Or at least, that's how I've chosen to define the effects of lonely longevity on persons trying (and usually failing) to remain sane. If I was publishing the "uncensored" Realms, in fiction, most of my liches would be desperate to have physical relations with adventurers, not kill them. Think about it.
I'm well aware that many gamers, reviewers, academics, and persons with only a casual understanding of fantasy roleplaying games have labeled me as some sort of pervert or (at best) immoral "dirty old man" for holding such views (strange, that 'old' bit, considering I was examining these issues and settling on this particular viewpoint when I was about fourteen), and of course the shortage of centuries-old real-world people to examine makes the point moot, but I'd like more folks to consider that situation and come up with alternative desires and drives that might dominate such long-lived characters (pure power is one, striving for immortality at the cost of humanity through undeath is another the game rules present to us).
Hi, Arthedain. Certainly George's (ahem, Mr. Martin, but he told me call him "George" when I sat drinking and talking with him at the 2003 Worldcon) nobles echo the behaviour of SOME nobles in Faerûn (such as those of Cormyr, Impiltur, and along the Sword Coast. "Arranged" marriages do foster (but not necessarily cement, because some families are quite internally fractious, so married-off daughter X may very well side with her husband's kin against dear old controlling mummy or daddy) political alliances.
As for the marriage between Lord Mourngrym and Lady Shaerl: the Rowanmantles quite openly and heavy-handedly THREW Shaerl and her sister at the lonely, far-from-Waterdeep Mourngrym in open and obvious hopes of gaining a toehold in the Dales, a move supported by the Crown of Cormyr (read: Vangerdahast) because he saw it as a way of extending Cormyrean influence into the Dales (first: make Tilverton a protectorate, then marry into shared control of Shadowdale, and - - the riches of the Moonsea are within sight and grasp!). Shaerl and Mourngrym both saw what was going on, but a marriage came about because they genuinely fell in love (and Shaerl politely told Vangey, via various War Wizards, to tluin off and get lost; she wants Shadowdale and Cormyr to be firm friends, but she's having no part of Cormyrean control, paternal and friendly or otherwise). So this was no 'hidden' plan: Vangerdahast didn't want to be subtle - - but he didn't succeed, either.
Now, as for your "general impression of the nobles and noble families has been that they feast, hunt and enjoy themselves (almost) 24/7 (or 24/10 to be precise), and that the heads of the families don't arrange marriages."
Not so. The 'idle rich at play' stereotype is just that: a stereotype. Someone in most noble families has to be loyal to the ruler, someone has to be good at war (or at least command), and someone has to be a shrewd investor: or the family will fall from grace and power in a hurry, and be exterminated, stripped of noble rank, or reduced to a handful of mere pawns in the hands of others. Many nobles like the wider Realms to THINK they spend all their time hawking, hunting, drinking, dallying with each other's spouses, attending or throwing debauched revels, and pursuing eccentric hobbies - - but most of them do so a few evenings a week, at most. YOUNG nobles, with nothing to do (because they're not trusted with any family responsibilities yet, as their elders watch to see what sort of people they'll turn into), now... many of them DO carry on like that all the time (or as much of the time as they can remain conscious and out of custody).
Moreover, in most noble families, even if the head of the family doesn't arrange a marriage (and they DO, whenever they have offspring not strong-willed enough to stand up to them, or a monarch meddling), their approval is needed in most cases unless the errant son or daughter is willing to elope, flee far beyond reach, and renounce all family wealth and favour if not name and title (and they are often disowned in absentia; when the furious head of the house dies, they may or may not be 'allowed back in').
It's always a mistake to try to relate conditions in the Realms too closely with the past history of our real world, as I've said many times before, but yes, in our real world arranged marriages were quite common in our Western World, and ARE STILL quite common in many other modern-day societies (many Muslim societies, most Hindu countries, and many African tribal cultures, to name just the first few that come to mind). So yes, colour yourself unsurprised: such practises are indeed common "in at least some parts of Faerûn." And yes, you correctly cite a published-early-on example of mine in this regard.
Shandril's Saga depicts a wedding performed by a Tymoran priest where the priests holy symbol drags the couple away and into the sky as a show of Tymora's specific blessing on the ceremony or some such.