Sims 2: Babies For Same Sex Couples
The Sims franchise has always been egalitarian regarding same sex couples. Unfortunately, there is no way around the simple biological fact that boys can't make babies with boys, and girls can't make babies with girls. Here are the options, if your same sex couple wants to have a baby:
1. Adoption, the obvious choice.
Have your Sim pick up the phone and call Service -> Adoption Agency. If your Sims meet the adoption agency's requirements, a youngster will be delivered to them the next day. When you adopt, you get to choose between Baby, Toddler, or Child, which is nice!
2. Plantsim Spawn
If you have the Seasons expansion pack installed, you can turn one of your characters into a Plantsim. It's not easy to turn your Sim green, since the change is related to the number of times your Sim sprays his or her garden plants and fruit trees. And you have to wait for the plants to need spraying, you can't just send them out there to spray things willy-nilly.
Once your Sim has been turned, they have the option to spawn a Plantsim baby.
3. Alien Abduction (Men Only)
When your male Sim is abducted by aliens, they may return to your lot having been impregnated. The pregnancy proceeds just as it does for female Sims, with morning sickness and everything. At the end of the pregnancy, your Sim will find themselves the proud parents of an alien child.
Side note: the alien child's other father is the Pollination Technician, from whom the alien child inherits its "distinctive features."
4. Tombstone of Life and Death
The Tombstone of L&D is a cheat object which you can make appear with the boolProp cheat. Once you have spawned the Tombstone from your Sim, you can use the Tombstone to then spawn a child from your Sim.
This is an excellent way for a same sex couple to get a "true" child, but the usual boolProp caveats apply. If misused, boolProp can trash your entire game, so use this option with extreme care!
5. Infidelity
It's not a savory thought, but it can be kind of fun - the challenge is not to get caught! If you decide to go this route, you will spend a lot of time shuttling your Sims on and off your lot, to keep them from learning about the hanky panky going on.
Female couples have it relatively easy, because they can carry on a simple affair. (You will probably want to sign up the other Sim for a job first, to leave the house free for a set period on workdays.)
Male couples will have a harder time, because somehow the female party will have to stay on the lot long enough to carry to term. This means that you will have to Propose -> Move In, so that she lives on the lot, and keep her apart from your Sim's boyfriend/husband. One suggestion is to create a second house (more of a stand-alone room or shed) on your lot, and lock the door. Think of it as a love nest!