Adoption fills more than just the desire to have and raise children. It does so in a way that fulfills the need for certain children which have no families to be raised by those who actually want them. The desire to adopt may have nothing to do with whether or not a couple can produce their own children, or even if they already have children or not. Many people adopt because they feel a desire to help a child. To give a child a loving home. One they know they can provide. This is true for heterosexuals and homosexuals. It is not an orientation related decision, it is a human being related decision.
Furthermore, as to having a mom and a dad to fill some supposed need for certain gender role models, that's absurd. As single parents have been raising children by themselves forever. Role models, ideas about gender, and so forth can and are supplied by various people in the life of a child. Just because a little girl has two daddies doesn't mean that there won't be women in her life to help her through certain female related things as she grows. Families include aunts, uncles, grandparents, brothers, sisters, best friends of the parent/s, and on and on and on. Even within heterosexual couples one parent may not be available often or even comfortable with handling certain things and may defer to someone else. OR, one parent may simply not be a good role model or person to have in their life period. Just because a couple is heterosexual does not mean that they are automatically a better choice for a child to be with. People should be judged for who they are and how they care for and love a child, not for which way their orientation happens to go.