My RP characters have killed lots and lots of people, and I don't feel like a murder yet. so I don't think characters having sex is the same as, say, the players having sex. 

Though I keep plenty of distance between myself and my characters. I'm usually not living vicariously through them, doing things that I'd like to do. If you look at the other person's character as their player, then your feelings of guilt will likely only get stronger after your character bumps uglies with them. Worst come to worst, you can talk to your partner about it. 

[i]Who knows, they might really be into it. [/i]