I thought it'd be cool if once you came inside a girl the next time you loaded the game she would be pregnant. I then thought about the easiest way to implement it and this is what I got.
First at least 3 ways to not get pregnant. Condoms for the men that turn off the function. Next, a toggle for a girl to be on the pill which would also disable the function. Then finally a single sex position with the man ejaculating on the girls belly or something for "pulling out".
Then there would have to be textures for each girls pregnant morph, and finally the mechanic implemented.
I imagined one new button being added above 'cum now' that would be 'pull out'. Pressing 'pull out' would switch to an animation of the man stroking himself. He would cum once the meter fills or when the player pressed 'cum now'. If you don't press pull out, wear a condom, or have the girl on the pill, the game edits the females XML file making her pregnant, with pregnant textures, however this would not take effect until the next time the player loaded her file.
Anyways, that's my idea, and thanks for letting me share..... In possibly way to much detail.