Fantasy
Ya know, I'll never understand this...
It's one thing to swing and enjoy our sexual adventures between consenting adults, but it's an entirely different thing to even think about innocent children in these sexual adventures - born OR unborn. It's fucking sick and its fucking disgusting. It borderlines pedophilia in my opinion.
Even I have standards.
Well, for my own part, I feel fairly confident in my non-pedophilia eventhough I do like the pregnancy kink. Heck, I don't even care much for the ubiquitous teenporn even when they're 19. Girls (or boys for that matter) don't really do it for me, I like women.
So why the hots for pregnancy?
Speaking only for myself, I suppose it's rather superficial - I think women on average look more beautiful when they're pregnant. Beauty is often linked to face and body symmetry, and pregnancy does not necessarily screw up this symmetry (in fact, I often find that the body symmetry is enhanced in pregnant women). Add to that the fact that guys like me, who love womanly curves, find in pregnant women even morecurves. I suppose it may be something rather basic and biological - the proven fertility of women makes them beautiful in the eyes of men, or whatever.
Now, of course, some things may need to stay a fantasy, and I sincerely believe that getting pregnant just to satisfy a kink is a spectacularly bad idea. You should have babies because you want them and because you can do the job of raising them, not because you want to get off. But that's another discussion outside the 'fantasy' realm, really.