My wife is pregnant now, and I think she is very hot.
I have also previously hooked up with a pregnant friend and had no problems there either. The body is a bit more lush, they are hornier than normal, and there is more natural lubrication. Whats not to like?
If your having sex and thinking about the fact that there is a baby inside, your not turned on enough to be having sex yet. Rational thought is not required.