Well when I did it, I clicked on the sim and options came up. You know like be funny, be romantic or something, but at the top there should be another one that says "propose marriage" click on it, buy a ring, then you're engaged. Then be only romantic and you'll get married. Keep being romantic and then you should eventually be able to have a baby. If you could not do anything like that, then there is a bug. Make sure that you've got the updated version though.
Hope this helped!