Forum:Romance (The Sims 3)

A sim I created is best friends with another, single sim. Both are adults. Their relationship bar is totally filled and they interact with each other romantically everyday. They've moved in and even had a baby together. However, their relationship status never changes from "best friends." How can I turn them into romantic partners? I want to get them married and one of them is nearing being an elder, so I really need to know how to bring their relationship to the next level. Thank you!