My boyfriend is the same way. I think the reason is because it feels different to them.& it's not as fun because they have to think about not going so rough and etc. they don't wanna do something that isn't gonna satisfy the both of you. Once I'm not pregnant he goes crazy lol. It'll pass once baby is out. Don't worry to much about it now...
Don't build his temporary lack of interest into a big drama of you "feeling unwanted and neglected." There is more to relationships than sex.
To test the idea that he is getting cold feet on the relationship (which it sounds like is what you are worried about), gently let him know you are still interested in sex, and ask if he will sometime tell you for sure what is going on that he is not, and then leave it at that.