Well, this is certainly a little more than just a comment about another pretty girl. I'm with Maddie on this one. Has he ever said anything like this before where you might think he has a fetish about pregnant women? Or was it just a comment out of the blue?
As a man, we do have sexual thoughts about almost every member of the opposite sex, but, as you said, the thoughts leave our head almost immediately, especially if another distration pops up, like a football game. And you sure don't tell your SO about how attractive someone is and how you'd like to "do" her!
But, maybe he was caught off guard or something to make him tell the truth and blurt out a very inappropriate thing. Has he ever said anything like this on other occasions?
It's almost as if you say something you're thinking and then say. "Did I just say that out loud?" Even after talking with him, it is evident that he does not attach the same importance to this creepy comment as you do. Luckily, you are not yet married to this guy because you have the option of dropping him (always a tough choice when love is involved) or of staying and reevaluating whether he is marriage material for you. Over two years together is a long time to get to know someone.
At any rate, the comment was completely out of bounds and it sounds like you are dealing with it. Ask yourself, if I love him, and, if we were married, would I let this remark go or be very concerned about him hitting on my sister in the future? It's your future and you have to decide whether this guy has the character to be a good husband and father to your children. If you appreciate honesty, then be honest with yourself about him.
By the way, nice egyptian hyrogriphics for your username.