The discussion on subversion in slash led a lot of places for me - both places on livejournal and places in my head :-). One place it led is thinking about motivation for sex. Here are some of my ideas on the subject. I'd love to hear others' thoughts, as well.
I'm a firm believer in treating sex in fiction like we treat all other human activities.
A sex scene should be in the story because it serves a function in the story. Sex is great for a lot of purposes:
* it can reveal something about character
* it can reveal something about how two (or more) characters relate to one another
* it can advance the plot
* it can explore themes
* it can add verisimilitude to the characters' lives by showing them doing something our readers all do and can relate to
* in counterpoint to above, it can be presented in a way that feels alien and distant from our readers' experiences, and signals something about the characters that way
* it can have an emotional effect on the reader (not to mention a physical one).
So, those are some of my motivations as an author for hooking my characters up. But what are their motivations? I think that's something an author ought to think through very carefully before writing the sex scene. I think exploring what's going on in the characters' heads in the characters' beds :-) is a whole lot of what makes sexually explicit fiction (be it slash or something else) interesting. And it doesn't necessarily happen in an expository way, with the author telling you their thoughts. In fact, I think it works better if it's not so direct. Sometimes you glean thoughts from actions or from words (I love dialogue in sex scenes). Sometiems motivation becomes clear from behavior before or after sex. Sometimes it's just implied. In all cases you've got to know why they're doing it - even better than your characters know their own motivations - before you can write it.
A lot of people seem to divide sex into two categories: fucking and making love. The former is just being done to meet the biological urge, the latter as an expression of love. I don't think it works that way in real life, and I don't represent sex that way in my fiction.
I think people's motivations and reasons for sex are so much more complex than a simple dichotomy would suggest. I think that's true even for a one-night stand or a brief encounter in the backroom of a bar. Even in a casual sexual encounter, choices are being made: the choice of partner, the choice to have a partner rather than just self-pleasure, choice of sex acts engaged in and not, and how you do it for any given act.
There are reasons behind those choices, even if the people engaging in the acts can't articulate them and/or don't fully understand their own motivations. We as authors can make interesting fiction out of exploring and engaging with the complexity of motivation implicit in even the most uninvolved, casual sex.
Similarly, in a loving, long term relationship, people have sex for lots of reasons, which differ from time to time. Some of their motivations they may not be aware of. Let's look at an example. You can have a situation where a couple is having sex and Partner A is:
* trying to exert dominance at home, since he feels like he's been stepped on at work
* wants to relax after a hard day
* trying to find something to do with his lust for the next door neighbor and displacing that on partner B
* trying to get over his lust for the next door neighbor and focussing on Partner B to do that
* worrying about money, since his job seems shaky and wanting distraction from that worry
* wanting to talk to Partner B about his bad day and thinking sex will loosen him up enough to do so
Meanwhile, Partner B:
* is kind of uneasy about the way Partner A is talking about and looking at the next door neighbor
and feeling reassured that he wants sex because that means all that next door neighbor stuff was probably just imagined
* wants to go on an expensive vacation in a couple of months and thinks Partner A might balk at the
spending, but if they have sex first he'll be more compliant
* has been suffering from insomnia lately and finds sex in the evening makes for a better sleep
* has a big presentation tomorrow and absolutely needs to get a good night's sleep tonight, because
there's just so much you can do with caffeine and adrenaline
So, all of that is going on at the same time and a lot of it is not even at the conscious level. And, yes, they love each other but the sex they have has so many more dimensions than just "making love" or "fucking". I actually think sex is rarely, if ever, solely an expression of love. There are lots of things we do for others solely for love, but generally when someone has sex - even with a partner s/he's in love with - there's an expectation of personal gratification, too.
I am not suggesting that the author explain everything I put into the bullets above, and certainly not suggesting s/he explain it during the sex scene. But I do think the author ought to know a lot more than s/he discloses about characters, and sex is no exception. If I were writing the scenario above, some of that information might be given to the reader in scenes previous to the sex scene (e.g. you might see what happened to Partner A at work or have been privy to Partner B's thoughts seeing A flirting with the next door neighbor). Some of it might be revealed after the sex scene (B might broach the vacation topic in the post-coital glow and get an earful of A's money worries). Some of it might be implicit in the scene itself, and some of it may never be revealed. I maintain, though, that knowing all of that will inform the sex scene and grant it a verisimilitude that "they're in love" or "they're just doing this to get off" can't.