I've kinda had the same thing happen to one of my sims.. When you see the bar when they are talking do you ever get to the stage of "very alluring", or "extremely irresistible"? Because my sim couldn't ask to even go steady for awhile, but then it I got the other sim to feel like I was being extremely irresistible and then they could go steady.
You can have romantic interests and a partner... are you sure the baby's father isn't partnered? I know when my character is partnered, I can't go steady with someone else, propose, nor marry. Those options don't show up.