-When both parents are not busy, have the father sit and watch television until he is very relaxed. Then, tell the mother to give him a back rub. Keep doing this until the father enjoys the back rub. Note: You may get denied at first, but keep trying. After successfully given him a backrub, kiss him. He should fall in love; if not, keep doing this until he does. Then, have them take a Jacuzzi together. Have them sleep together that night. If nothing happens, have them sleep together the next night also. Eventually they will have a baby. You can also do this the other way around, with the father starting it.
-Purchase a Vibratomatic love bed and instruct your Sims to "Play In Bed". If they play in bed every day for several days in a row, they will eventually have an accidental child, and no "Would you like to have a baby?" dialogue box will appear. Note: This only works for opposite-sex couples.