I'm personally not fond of sex scenes unless they serve the narrative in some way. It could be for characterization, connections to overall themes present in the story, but it needs to have some reason to exist. I dislike it being written in just for the sake of being there, outside of pure wank material, in which case my standards will be much lower.

It's actually funny, because I've recently been reading House of Leaves by Mark Danielewski, and there's quite a bit about the sexual proclivities of a few characters in there. I haven't found it written about in lurid detail outside of maybe a few paragraphs so far, but I haven't minded it, since I feel that the descriptions of physical intimacy when they do appear contrast well with the theme of isolation that's heavily present throughout the story. I feel there's more to it than that as well, but on a base level, that's enough to justify it to me.