I have wondered about this question before. There are people in SL who have no problem with it. I am not sure how I even feel about it since it's still kinda new for me and I am currently single in RL and in-world.
I recently came across this posting in New World Notes. Blogger CeNedra Rivera recently conducted a survey and asked if Second Life sex with someone in-world besides your RL partner constitutes cheating. The survey results can be found on her website.