Because since there are two sexes for it, two sexes must be right. Since two sexes is right one sex must be wrong.
Tell us how one sex can be right when there are two for it?
You can say that two sexes are right when the goal is sexual reproduction, but homosexual behavior has no possibility of sexual reproduction, so any argument against homosexuality based on a situation in which the goal is sexual reproduction, or a situation that has the possibility of sexual reproduction, can't apply; it's the wrong situation, it's a category mistake. (If you want to argue that sexual behavior should only be for sexual reproduction, then you're welcome to make that argument.)
The more precise word to replace "right" would be "necessary:" two sexes are necessary
given the possibility or the goal of reproduction.
Therefore, you can't make a logical argument that one sex is wrong because two sexes are right because it's merely that two sexes are necessary
for reproduction, and reproduction doesn't apply in the case of homosexuality.
If you mean that two sexes are right as a pure judgment, then that's not a logical argument, it's just an assertion. I don't know what else "right" could possibly mean here, other than an over-statement of a better word, "necessary," or a pure assertion.