Gays are not denied marriage - there is nothing stopping gays from marrying opposite sex partners.
It's true that opposite sex marriage is given a privileged position, and this benefits people who want to engage in it. It's also true that purchasing a home is given a privileged position, and this benefits people who want to buy a home with a mortgage.
Thus, denying subsidies to renters (unless they go against their preferences) is discrimination against people who prefer to rent, just as denying marriage to gays (unless they go against their preferences) is discrimination against gays.
It's true that opposite sex marriage is given a privileged position, and this benefits people who want to engage in it. It's also true that purchasing a home is given a privileged position, and this benefits people who want to buy a home with a mortgage.
Thus, denying subsidies to renters (unless they go against their preferences) is discrimination against people who prefer to rent, just as denying marriage to gays (unless they go against their preferences) is discrimination against gays.