This means fnding another Sim is going to be the simplest route. Probably not what you were wanting, but it can be done. With 'Seasons,' Male Sims can get abducted and impregnated by aliens.

For a gay couple, you'll can either go with the 'Seasons' expansion pack or use more conventional methods and then jump through hoops to get the baby added to the household. Lesbian couples will have an easier time getting pregnant within normal game mechanics and will also not have to go through the hassle of getting the baby into the household since the baby goes with the mother's household by default. If you don't understand why, then have someone explain the birds and bees to you. If you want your Sim to have those experiences, then you're going to have to go find a partner of the opposite sex.

They can adopt children, but do not get to experience the joys of morning sickness, cravings, incessant wishes, tossing baby showers (well, hosting parties where you can announce your pregnancy), and emergency trips to the delivery room. Barring cheats or mods, gay or lesbian Sims cannot produce children.