Well, for starters, being black is a simple variation in skin pigmintation. A person has no control over being black.
Similarly, a person (supposedly) has no control over being homosexual / bisexual. However, the state of being is not what is sinful. A person being gay is about as important as a person being black, or white, or having brown hear, or having green eyes. It is simply a state of being with no moral implications.
What the homosexual community is arguing over is the “Right to marry.” They claim that there is no difference between a heterosexual marriage and a homosexual marriage.
All you have to do is ask one question.
“Can two men naturally produce a child through homosexual acts?”
Ignoring the immorality of homosexual acts, this is really your best bet at a natural law argument. They cannot, and therefore cannot be equal to a heterosexual marriage, which is capable of producing life, and extending society (which, incidentally, is why heterosexual couples receive tax and death benefits, not just because they are married and in love, as the gay agenda people would proclaim)