Your attitude is not crazy, it's quite common from things I've read. Now that your wife is showing, it changes how you are seeing her, from a sexual being, to a mom...
Check with her doctor if you want your mind put at ease, but sex during pregnancy is perfectly safe even up til the 9th month (in fact then it's been said that orgasms can reduce both the amount of time in labor and the intensity of the contractions)
(THere's a line in Sex and the City about sex during pregnancy, and well, what if he's really huge, won't he hurt the baby - -the response back was- well where do you think dimples come from? - -Not exactly true - but dimples are very cute
