I have the Teen Pregnancy mod, and I got my boy sim(Elijah) engaged to his girlfriend(Sandi), and they are both teens, but I can't get them to even Woohoo because it says "One of the sims is a teenager!" whenever I hover over the "Woohoo" action. Do they need to be fully married? Please help me!

I had the same.problem. What you need to.do (assuming this is NRaas) is to set "Check Story Progression for User Directed" to False. Also, you have checked that all the relevant restrictions have been lifted, yes?
Hope that helps. If not, I can look at my game in a few hours, unless someone else comes along

Is it Twallan's mod? If so, you need to change the settings so that it'll allow you to have teen pregnancy. Oh, and you need teen maternity CC too.