It is regarded as modern people as a foothold for marriage. Where does that leave people who are not quite either male or female - at least according to the definitions ordinarily employed? Opposing the inevitable marriage equality is a waste of time, money and energy. If a complementary relationship hinges on procreative sex, are these relationships unnatural? When all of the arguments that gay marriage is an intrinsically bad fail, religious conservatives move to argue that such marriages will somehow infringe on their own civil rights. They are worried about how we can earn enough money for our family or we are unemployed.
Whether a marriage is between a man and a woman or two people of the same sex, the reasons behind getting married are the same. This had nothing to do with homosexuality. And how?
There are, of course, the legal, property, and financial benefits of being married.