I wanted to write a story where a married guy goes to Japan for vacation and ends up trapped in a government run, free use, breeder program set up to combat chronic low birth rates. It specifically targeted tourists to encourage genetic diversity. Maybe he gets released on the condition that he impregnates a certain number of women. Would he get a card that gives him a free pass or would he need to win them over on his own? Would his wife become his partner in crime to make it happen? I ended up not having enough time to make it happen, but I wanted to take a crack at it again. I think that the concept is pretty solid.
Seems like there is room for a lot of possibilities. If was the main issue, local women could be encouraged by getting government benefits for decades when they have a mixed baby. (There might also be a genetic reason for the low birth rates, so the locals might get pregnant more likely from the tourists.) I guess it might also work the other way around. So while the guy has to knock up a few locals, the wife might have to have a mixed baby on her own to get off the hook. That way, you could introduce a dynamic between the two that might develop one way or another. They could also try to flee with the help of locals who might then exploit their situation even more.
Something to think about: if all they need him for is his sperm, why not just have him fill a cup and send him on his way? There should be a reason why he needs to deliver it the old fashioned way, and that could open up plot possibilities.
What are your goals with this story? You could do some research on the actual issues plaguing Japan's birth rates and make a satirical commentary of the government's efforts to combat it, think of this like some well masked social commentary. If you are just writing standard smut with no backdrop purpose you will burn out and realize you just wanted an excuse to write a sex scene between a foreign man and multiple Nippon women, and end up dropping your work half-way. The captivity situation if written well could be a reflection of the draws and reluctance of a group of foreign married men in the real world who find themselves stuck in Japan fooling around and getting enthralled by Japan's red light districts or something like that. The local men are not chosen as frequently and it could be a reflection of a societal culture or liking towards westerners due to biases. Just throwing it out as examples.
