If we accept that sexual relations, companionship, love, child-rearing, and shared responsibility are the pillars of marriage, and we reject any legal unions that do not aspire to uphold the marital institution, then along with homosexual relationships, we would have to exclude asexual relationships, relationships between couples with no child-rearing aspirations, and relationships between couples with no capacity for love from the marital institution.
Also, there is no reason why a same-sex couple cannot support all of these institutional pillars. Same-sex couples can and do rear children in a loving environment.
Finally, the notion that all of these components are prerequisites for marriage is debatable. Marriage means different things to different people. For some, it may mean love, for others deep understanding. For some, sexual relations are a key component of marriage, for others, they are absent. The definition of what marriage means to that specific couple should have no influence on their legal ability to wed.