In Sims 3, why can't my sim propose?

I have a young adult male sim on my Sims 3 computer game. I have a rich, full family. One of the boys started dating a sim I created on edit city along with her family and moved them in with a previous generation of my family. He knows all of her traits and they've been together and have a full relationship bar since teens. Well now they are young adults and halfway to being adults. I would make him be romantic until it said "Emmie thinks Chase is extremely irresistable" and wouldn't even give the option of "Propose". They had their first kiss and went steady (became a couple) in their teens. He only has four traits though, like I chose one trait when he was born or something, so I don't know if that has anything to do with it... Anyways, so I said "try for baby" hoping maybe he could propose so I can get rid of this frigging family! Nope. But now, she had a baby and I feel bad cause he can't live with the kid, just stay over at her house. So what do I do? Any ideas how to make him propose? (Oh, and his traits are brave. workaholic, neat, and athletic) Or is this a glitch? Should I just give up and see if one of the other boys (they're all boy kids) can get married? I just can't deal with four elders and a cluttered family anymore! HELP! ... Thanks! :)