Same-sex relationship
In the Sims, every sim is "bisexual", he/she can have sexual intercourses with any gender, if young adult or up. Strangely there aren't any homosexual premade family in Sunset Valley or in Riverview. We can think that every sim is heterosexual by default, except the one of the player.
Difficulties of a homosexual couple
A homosexual couple can't have natural children because when in the bed they haven't the interaction "Try for baby" even if lesbian. Except this they are considered like heterosexual couples; they can wed.
Possibilities of a homosexual couple
A homosexual couple can adopt, in this case the two parents are considered as fathers (homosexual males) or mothers (lesbian couple). Sometimes the wish "have a baby with [name of Sim] will appear. In this case the points are earned only if the sim asked for make the adoption.