not being a dude, nor straight, it's something I don't particularly think about HOWEVER, I do believe there is an old, old OLD preg story somewhere (was it Billy McFred?) that if I recall, involved what you described.
I have two fantasies that come out of this. The first is as the O.P. described. Everyday I pour gallons of cum out of my unrealistically huge and full balls into a woman or women until they are a few months along.
The second one that have is a different take on it where the male and/or female gain or lose weight due to a reaction or something like that. For instance, a man is eating out his woman (yes the phrasing was intentional) and because of some new vitamin or something that she's taking, her cum is causing him to have a voracious appetite and/or gain weight. Or a female that when cum enters her body, especially orally, she tends to gain some weight, or you can even expand on that scenerio and have her man lose the weight that she's gaining. She's sucking him dry sort of thing. Something like that. I find those things way more erotic.
atl feeder wrote: I love the idea of this fantasy. My idea is to hook up and enema while having sex and when you get close to cumming turn it on and start to fill her up. Just the idea turns me on. Watching her belly expand as you make love. Any woman game?
I've actually thought about that too, but instead of an enema w/ liquid, using just air. I think it would be great...and a lot cleaner if there was a spill...
this is also such a tun on for a guy, to imagine him continuously pumping even more cum into her, watching her belly expand and inflate with every pump. seeing her boobs get bigger, her ass, her hip etc until he pulls out and shes releases it everywhere
I find it personally entertaining the number of female responses to this fetish. It's not bad, just unexpected for me.
When I was in middle school this idea just popped to mind; no outside influence. I eventually found hentai that featured cum expansion, but very few matched the fantasy. The way I had fantasized was being able to pump a girl up so full she would be to big to move (looking like the thousand pound girls in some weight gain art) the whole thing being so pleasurable for the both of us that we'd go again, ballooning her even bigger.
In more recent years, the idea of the cum itself being super fattening has had an appeal. There's a weight gain story I found on the dimensions website that features this idea with a BHM losing weight and his wife getting fat.
I love this fetish as well. I do like to imagine it combined without the refractory period for us guys, so I can keep going and going. I also like it with the ladies gaining weight depending on how much cum is taken from me. My brain does some weird things too. Where sometimes I imagine the lady gaining weight above the waist when she goes down on me and below the waist when I'd ejaculate during sex. Sometimes conversely, I gain weight from the ladies as well. Gives a new meaning to a Hungry Boy.