Angel and Buffy only do 'it' the once while he lives it in Sunnydale because of the fear that he will turn into Angelus as a result. Buffy and Angel do get busy with one another in an Angel episode called 'I Will Remember You' when Angel is made human again because of being infected by demon blood. You should check out the discussion thread in the Angel season 1 section. Angel and Buffy spend season three trying to go back to holding hands but find that it is not really possible, so that and his need to find his own destiny cause him to move to LA.
A little aside, i've seen some discussion of the theory that if Buffy and Angel were to have sex again, then maybe it wouldn't mean that Angelus would arise because it was the fact that Buffy was a virgin that caused the perfect moment rather than the actual event. This is because of something that Angel says to Dala in Season two of Angel (epiphany, i think though don't quote me on that) and also I've wondered if Angel's perfect moment would even involve Buffy post season three of Angel when Connor becomes the most important thing in his life.