Tendencies toward alcoholism, obesity, and violence are now thought to be genetically influenced, but they are not good behaviors. People born with tendencies toward these behaviors have to fight hard against their natural temptations to drunkenness, gluttony, and physical rage. And since we are born as sinners into a fallen world, we have to deal with the consequences of the Fall. Just because we're born with something doesn't mean it's normal. All of us have effects of the Fall we need to deal with.
Marriage is a legal institution in order to offer protection for women and children.
Women need to have the freedom to devote their time and energies to be the primary nurturers and caretakers of children without being forced to be breadwinners as well. God's plan is that children grow up in families who provide for them, protect them, and wrap them in security. Because gay or lesbian couples are by nature unable to reproduce, they do not need the legal protection of marriage to provide a safe place for the production and raising of children. Of course, a growing number of gay couples are seeking to have a child together, either by adoption, artificial insemination, or surrogate mothering. Despite the fact that they have to resort to an outside procedure in order to become parents, the presence of adults plus children in an ad hoc household should not automatically secure official recognition of their relationship as a family.