Could you expand on the question?  I'm having trouble figuring out what exactly you're asking after.

Do you want to know how people determine what is right and wrong with some sexuality in particular in a 'religion driven' society?  What society are you talking about that is religion driven?

Here's a broad set of answers, I hope they end up relating to your question:

So far as any sexuality goes and religion: If religion is what guides one person's morality, that's keen. But religion does not drive all of society, even in theocracies.

As for right and wrong in sexuality: Keep it safe and keep it consensual and it's morally acceptable.

Without going into too much detail...Atheism is extremely liberating regarding sexuality. Once you are able to conjugate in the nature, manner and frequency that is natural to you without bearing religion induced guilt...well, what can I say?...Life doesn't suck.

 Conor, when you read my stuff on sexuality, you'll find that most of it is very dry and scientific, or dry and historical.  There's a reason for that.  I tackled the easy parts first.

Despite how obvious it is to some people that humans are "naturally" this way or that way in today's society, it's not as objectively easy to demonstrate.

Human brains were not built (by blind evolution) for post-industrial information age dating.  We have more options, more privacy, and more travel radius than we ever have before.  Many of our sexual emotions were built when we had maybe a half a dozen mate choices, and anything we did was pretty well known by everyone on earth that mattered -- our whole tribe.  While our ancestors may not have recognized the importance of sex with regard to paternity, our genes did, and they programmed us with sexual possessiveness and jealousy, but they also programmed us with good things, too.  Especially when women have an orgasm, they get a rush of natural drugs that cause a physical addiction to their partner.  Even without an orgasm, most women, and many men, are very likely to develop a completely natural attachment to their partner and jealousy towards anyone who tries to steal them.

In this age however, I think it's important for us to take a lesson from other less controversial areas of human nature.  To quote myself:

Our natural drives for sugar, salt, and fat, have been the catalyst for a fast food industry that is literally killing humans by the hundred thousands.  Clearly, just because our emotional desire for certain foods is natural, it's not necessarily good.

With sexuality, though, it's tougher.  For one thing, our emotional reaction to being denied a big mac is (hopefully) much less severe than being abandoned by our lover of ten years.  Some people will argue that human sexual emotions are so strong that our desire for monogamy is equivalent to "morally right."

This is special pleading, of course.  The strength of our emotional convictions is not a barometer for the strength of any moral imperative, nor does it provide any foundation for the formation of any worthwhile moral system.  Just think for half a second and you'll see why emotion is completely useless in any kind of moral system.  The whole point of morality in most cases is to do the right thing IN SPITE OF our emotional desire to do something else!

Here, we must pause.  Consider what I just said, and let the truth of it sink in.  Even good Christians will tell you that moral "systems" are designed to tell us what we should do even when our emotions disagree.  Think of a child who has no money in his pocket and really, really wants a candy bar from the grocery.  His mother teaches him, through a series of moral lessons, to refrain from stealing the candy bar AND to supress his anger at not getting what he wanted.

So, to suggest that the emotions involved in sex are so strong as to be considered immune from the very premise of moral discussion is absurd.  If we can look at our desire for sugar and realize that it needs to be tempered by reason, we can also look at our desire for monogamy and realize that just perhaps -- it's a remnant from another time in our evolutionary history, and that there is no hard fast rule that everybody needs one wife or husband.

Similarly, the desire to reproduce is extremely emotional, but we can say that reason allows us to conclude that some people ought not reproduce even though they have a very strong desire to do so.  This is reason over emotion.  It is the heart of morality.

Still, we are talking about morality -- not what is "natural."  This is where the second big hurdle comes in.  Is there any difference between what is natural and what is moral?  I suggest that there is not.  Bear with me, though, because that last sentence doesn't mean what you might think it means.  I view "natural" as a trick word.  Any behavior that a human is capable of is a "natural" behavior, simply because there's no alternative.  We were built by genes.  They are natural.  Without a supernatural soul, we can only regard everything as natural.

This is where that essay on murder and sugar comes in.  Once we remove "natural" from our vocabulary, we are forced into another paradigm.  Unfortunately, this is where a lot of Utopians have gone horribly, horribly wrong.  The French Revolution failed because politicians got a hold of the notion that sometimes the masses need to be forced into "Enlightenment Values."  Of course, very bad times follow when a government comes to this conclusion.

I suggest that only when we remove Utopianism from our vocabulary as well will we have any chance of achieving a very good society.  Perfect society is impossible for a variety of reasons which are not reasonably covered in a single blog post.

Having said that, we can look back at sexuality through the same lens.  Will most people opt for monogamous relationships?  Yes, I believe they will.  Most people will have babies "because it's the meaning of life" or some other such ad hoc explanation to try to rationalize the rush of emotions they feel after they become pregnant.  They will look at their mother and want to be like her out of the emotion of idolization and admiration.  They will see fathers as strong men of duty and responsibility.

There's nothing inherently wrong with any of this -- but there's also nothing inherently wrong with a nonmonogamous relationship either.  The lying and cheating that often go with monogamy are a result of the demand for monogamy, not an inherent part of nonmonogamy.  That is, people who know they can have sex with other people don't often lie about it.

This is all easy enough to put into a blog post on an atheist friendly site, but to sell this kind of thinking to the religious public requires a lot more justification, a lot more analogies, and above all, a lot more patience.

That's why all my stuff is focused primarily on science.