Honestly, because gay marriage will strengthen marriage as a whole, which is to the benefit of all of society.
An excerpt from an ethics paper I wrote:
The last major argument against same-sex marriage is that it undermines the institution of marriage. However, in actuality, “…preserving the ban on same-sex marriage will in fact weaken marriage, a little at first, but then more over time, by blurring its boundaries and eroding its prestige” (Rauch 86). Civil unions and partner benefits, as touched upon earlier, act as a “replacement” for marriage and, although meant originally for gay couples only, heterosexuals have also used these outlets in place of marriage. Allowing gay couples to marry would help prevent heterosexuals from “piggybacking on the movement” (90) and therefore help eradicate the acceptance of couples forming a life together without the structure of marriage. Furthermore, legalizing gay marriage would strengthen marriage by “renormalizing” it, so to speak (89). By including same-sex couples, it would reaffirm the idea that marriage is both the acceptable and preferable way for two people to build a life together. It would reinforce the idea that if a person wants the benefits of marriage, then they need to get married. All of the dangers, such as civil unions, cohabitation and unmarried parents, heterosexual dissenters say weaken marriage will only become more prevalent as homosexuals continue to live outside the fold of marriage and straight couples join them, leading to the eventual metaphorical “death” of marriage as an archaic and useless institution. This outcome, of course, is an extreme view of the situation that would take a long period of time to occur, but it is, nonetheless, possible.