Why is the "try for baby" option not popping up?

Make sure one Sim can get pregnant and the other Sim can get others pregnant. Check your household size as well; you can only have eight Sims per household, and if you already have eight Sims, you can't have a baby unless someone else dies or moves out. You should also check that the spot for them to WooHoo is accessible, and not blocked off by walls, objects, or locked doors. (If your Sims still can't Try For Baby after you've made sure everything should be working, try with a new save to see if that works, and check that you don't have any outdated or incompatible mods that could be interfering.)