I have been having that problem for ages now. But I think I have it figured out.
(BTW: Only bought SIMS 3 + expansion like a day ago, so don't judge my noobiness!)
Anyway, I have been eating life fruit as I wanted to be married with a child before I aged into elder. After a while of romancing, etc., marriage, be gf just wouldn't show!
Then I realized I had been eating life fruit for no purpose as he was a young adult and wouldn't be an elder anytime soon.
Could it be that young adults can't get into relationships like that besides a "crush" type thing?
Sorry if this isn't much to do with your question specifically, but I am desperate.