Adultery has long been condemned by Christianity and by Judaism before it, and in other traditions; I have no problems with this, but in a agreeing with it would like to look at what we can find as we reflect on this law: in particular, I'm interested in how the questions ``what is adultery?'' and ``what's wrong about adultery?'' reflect on how we relate to each other and to God. The drill bit of this string of thought is the scenario in which a man and a woman, at least one of them married but not to the other, develop a particularly strong friendship; a particular case is when one of them is not getting an important need met by their spouse (my own experience in this area from many years back includes general support of a friend who was ill and whose husband was fed up with supporting her); even if nothing definitely sexual happens, some people would say that, spiritually speaking, this situation is adulterous; while others would say it was just a good, important, friendship and probably valued as such by God. So here, we have a gray area, in which there could be overlaps (parameterized boundaries?), definite and immoveable dividing lines... and most probably both.
So what makes such a relationship right... or wrong? There's probably quite a bit of delight in each others' company and friendship -- as is characteristic of friendship in general -- and that is not in itself wrong (if it were, there could be no right friendship of a married person with anyone of the opposite sex, which I'm sure is not the case); and it's hard to believe that there is a dividing line of grounds of `amount of pleasure' -- could it be more like the kind of rôles they play in each others' lives that matters? What if the rôle is intrinsically good, but should be provided by the spouse but is being refused? (I'm happy to count sexual intimacy as being outside the gray area here -- I'm thinking more of general, affirmative and supportive friendship here) and what counts as intimate sexually anyway? For example, I've had hugs that I've felt quite uncomfortable about -- and that's not just ones accompanied by gushes about me being so much nicer than the hugger's husband! And yet some hugs given with equal or greater warmth have felt clearly OK -- fine, refreshing and wholesome and I've been happy to think that that friendship is fine by God too, as far as I can tell or have reason to believe.
Taking the prohibition on adultery in the context in which it was first written down in the Bible -- the Ten Commandments, several of which are about not using people as tools for your own fulfilment -- it could be read as:
and in clarifying this in the New Testament, Jesus says something that makes it clear that
Also, the repeated use of the idea of faithfulness and of joining together points towards monogamy as ideal.