Flickr photo by djwingsiaIt's not that they don't find their wives or partners attractive in pregnancy ... they do. It's just, they are having a little trouble with the concept, is all.
With the way guys feel about sex all other times (anytime, anywhere), Sam Holt of the Being Dad DVD series that aims to help guys be better birth partners and poppas, tried to figure it out. He interviewed hundreds of men around the world as to why some men don't like having sex during their partner's third trimester, and here's some of what they said (via Pregnancy.org):
1. They are scared of tapping the baby on the head. That's one of the reasons women have a cervix. Nothing gettin' through there.
2. The child might be a girl ... "Yikes! I can't do that with my daughter in the room. Now, a boy, that's a different story ..."
3. They are scared of hurting the mother. Refusing to have sex with her will hurt her more.
4. They are worried their semen might bring on labor. Prostaglandins in semen can start the process, but probably won't unless it's on the due date. Even still, I can think of at least one VERY OBVIOUS way to skirt this risk altogether. But for some strange reason, guys just don't like that.
See the complete list of reasons men won't have sex during pregnancy at Pregnancy.org.