Not generally, but I did cheat on my best friend's wife when she was pregnant, about a half-dozen times at that over the course of a month and a half. He was out of town at the time (he travels as part of work), and I dropped by after work to say hi. She ended up telling me that being preggo made her really horny, which was all the invitation I needed to fuck her (I had always wanted to, known her for like 10 years).
We've never talked about it and our relationship is pretty much the same way it always was. Sometimes I make eye contact with her and we both kind of get this little grin though, so who knows maybe we'll bump again but I'm not going to push it.