What does it mean
Basically instead of sleepwalking you have sleepsex. You're dreaming, you have no idea or recollection of what is actually happening. According to the House episode you can act quite normally otherwise but you don't care too much who you're sleeping with (the woman in the house tv-series was having sex with his ex-boyfriend even though the woman hated him. Weird.)
The mind is dreaming but the body is doing whatever. And this does not necessarily mean that the person is seeing a sex dream of that person or even dreaming of sex at the same time (I think. Maybe not. But this is a yaoi story so I guess it's okay even if it isn't exactly accurate.)
If you want more accurate information you should look up wikipedia or something. Hope this cleared something up!
Tyyy
I first came across the term "sexsomnia" in nemuri otoko to koi otoko, and afterwards I saw it in House M.D. tv-series in season 1 episode 17. So it is legitimate.