Create a family with a male and female adult that are of the same category (such as Virgo and Virgo) and move them into a house (or created house). Immediately after selecting live mode make them kiss approximately ten times in a row to set their relationship meter to be 100. If you continue kissing the female should ask to have a baby.
You can make a room especially for the parents with no doors. Use the move_objects on code and put the parents in a room with no doors. If you want them to have a baby, have them kiss a few times (not necessarily ten) and she will ask about the baby
Never let Sims choose their own social interactions. They make horrible decisions. The best way to build a relationship with another Sim is to talk until the relationship is at about 30. Compliment until it reaches 60, then hug until it is 100. If possible, do not fall in love -- it can bring down the Social motive when jealousy occurs. Just make friends and send them on their way.