Thanks for taking interest in my story idea, I haven't seen anyone do it before so I would like to think it's a unique idea.
The idea is that a man and a futa are married and they want kids. For obvious reasons they can't have children from each other and aren't comfortable with artificial insemination. So, they go to one of their female friends and convinces her to have sex with them to get her pregnant with their children. There would be different dialogue depending on who got her pregnant.
Alternatively, another idea I had was that again a man and futa are married and the guys birthday is coming up. The futa asks him what he wants and he accidently lets slip the desire he has to see her fuck a girl. The futa has never had sex with a girl and isn't sure about the idea. Throughout the VN, the player would have to convince her of the idea then find a girl to have sex with her. The end result would more than likely be pregnancy depending on if you convince her to not use a condom.
Let me know what you think!