Sex scenes are difficult to write. They are even more difficult to make interesting.

There are a number of reasons why they’re difficult to write: the writer is embarassed, the writer is paralyzed by the thought of his mother reading them, the terms for the parts in question are either overly clinical or inappropriately obscene or totally ridiculous, the sensations are difficult to describe, the actions are all more-or-less the same so it’s hard to make one kiss (for instance) seem different from another, the writer is worried that readers will think it comes from personal experience, or the writer is worried that readers will be able to tell that it doesn’t.

But there is one main reason why they’re frequently boring to read: not enough dialogue. By this I mean real dialogue, not "Oh baby baby baby." This is a corollary to the idea that sex scenes should express character. (It's also good if they advance the plot.) But while a lot of writers have gotten the "express character" part down, by showing how dominant characters have sex in a dominant (or sometimes surprisingly non-dominant) manner, and so forth, they frequently neglect to have them say anything.

While the moment of orgasm and the moments immediately before it do not lend themselves to coherent dialogue, people are quite capable of speech, if their mouths aren't busy with something else, at every moment leading up to the point when they get distracted.

They can tease each other. They can get distracted by outside events, like a phone ringing or too much light or not enough light. They can accidentally hurt each other, like by leaning their elbow on the other person’s hair. They can get into conversations, bickering, or arguments over what they're going to do with each other. They can reminisce about old times or other experiences. They can make discoveries about each other's bodies or what the other person likes. They can discover things they like that they never knew they liked. One person may have trouble getting into the mood, which the other may take as a challenge, an annoyance, or a sign of his or her own unworthiness. Especially if they're an established couple, they can have a conversation about something else entirely.

The single form of interaction that is most neglected in sex scenes is negotiation, which can be verbal or non-verbal. In fiction, people tend to cooperate perfectly and in utter silence and with no discussion of what they like and the other person likes-- and they do it all in perfect sync and with no hitches and with both of them getting exactly what they want. No one has sex like that unless they’ve been together for years and are having an especially good night, or one of them is faking it, or they're both telepathic. A little negotiation, before or during-- who's on top is an obvious one, and not limited to partners of one gender-- can be a great source of character, hotness, and humor.

On the page, sex is cheap, but witty banter is priceless.