I have a prostitute Sim and eventually she needs to pump out a baby heir to continue on the immoral legacy but, since it's necessary to have the commitment issues trait, she won't want to propose and marry (a good thing) but you have to be married to want a baby (a bad thing). I like doing what my Sims want and I only pop out kids when they want to and/or they're 4 days away from turning an elder.

Basically, I want my Sim to want to have a baby but she can't want it unless she's married but she doesn't want to get married but I still need to have her pop out a baby! I saw a "Marriage - Want Children" thing in S3PE but it doesn't make enough sense to me and reading (not skimming) 3/4 of that made my eyes die.

I hope that made sense.

I don't think there's a mod for this, but why do you need her to wish for it in order to get her pregnant? You can just have her get pregnant without committing to someone (true to her Commitment Issues trait) and then give her 5,000 liftetime happiness points (I think that's the number) via cheating afterwards, so it looks like her wish was fulfilled.
If you use a mod, it won't be "what she wants" anyway because it'll be an artificially induced wish.