Getting to the proposal is basically a matter of spamming your beloved with Romantic interactions.
It's not easy, though!
The first thing is, you need to have a player level of 10 or above. You can check your level on the bar at the top of the screen. If you are still in the single digits, then you will need to work on collecting XP in order to level up.
XP is the only way to increase your player level. You collect XP from almost any action, small or large. One of the best ways I have found to get XP so far is to buy a skill item for $1,000 and set a Sim to work. The skill items only drop XP, not Simoleons.
These objects have the additional benefit of boosting your Sim's career level, assuming you have coordinated the object with the career. For example, a Sim on the Artist career track should work at a painting easel. A Sim who works at the Science Center should use a telescope, and so forth. You can find the appropriate item for your Sim by expanding the bottom left-hand roll-out Employment menu.
As in The Sims 3, getting to the proposal is basically a matter of spamming your beloved with Romantic interactions. Except that in this case, you only get three choices. Luckily, other Sims never seem to get bored with your advances. Mix it up with some Friendly and Funny actions, just to keep that relationship up. Once their relationship is high enough, you will get the option to "Propose Marriage."
Here's a tricky thing: you have to buy a ring. The better the ring, the more likely your beloved will accept your proposal. If they refuse your proposal, you're out the money for the ring. Bummer!
Once your Sims are married, having a baby is simply a matter of buying a baby crib. Click on the crib in order to receive a delivery from the Stork Network. Woo hoo is not involved!
Have you been able to get your Sims married? Have you had babies yet? Can same-sex couples get married in this game? Can they have babies? This will be my next experiment, I am very curious to see how they handle the situation!