Hello ^^ Let's see if I understand things correctly. You want her to have a baby of each of the men? She can have a romantic relationship with both of them without marrying and without moving in. You'd have to ask her to move in and instead just move in the babies. At the moment, that's how the game mechanics work. As far as staying good friends, as long as each man doesn't discover the other romancing her, that's about the only way to keep the relationships from taking a hit. Now, that means each of them would have a romantic relationship with her, but it would fade over time.
If you use something like MCCC there might be other options there, but if you aren't using that, you'd have to use the current game mechanics. If anyone else knows a way around it, that would be great, but that's all I can think of.
So the logistics would be: one of them invites her over, starts a quick romance, enough to try for baby. Just make sure his male partner is gone. After the baby's birth, repeat with male #2. Then just let the romantic relationship with the female decay normally.