That's true, inasmuch as unmarried people get taxed more than married people. But that's not always the case. Sometimes it's the other way around.
But the reason for that is not because of discrimination against gays. It's because of discrimination between married people and unmarried people, and same-sex couples, just like all other unmarried people, are not married. While it is not legitimate for state and federal laws to base anything on this distinction, it's also not arbitrary and senseless, at least not when marriage is defined the traditional way, since it forms the core of the normal family unit in which children are raised following a model of a husband being the main bread-winner and a wife sacrificing her career to raise the kids. The only fix to this problem is to get rid of the distinction under the law and get the government out of the marriage business. Trying to fix a big government problem with a bigger government solution that will create new unforeseen problems, which is what happens when you broaden the reasonable definition of marriage as one man and one woman arbitrarily to include totally different kinds of partnerships, is a step backwards.
And the gain in freedom by some people paying less in taxes is more than offset by the loss in freedom by way of expanding things like Social Security to give spousal benefits to same-sex couples, and punishing private businesses for discrimination when they don't want to bake wedding cakes inscribed with the slogan, "Three cheers for fudge packing," and when an innocent woman like Kim Davis is put in jail.