As for men marrying men, heterosexual men can't marry men either. So, the law seems completely fair on this issue.
If we passed a law prohibiting bar mitzvahs, would that be discriminatory? It would preclude Christians, Jews, Muslims, and Buddhists all from celebrating a bar mitzvah. That law seems completely fair according to your reasoning.
Nope. She would have to be unmarried, not closely related and capable of giving legal consent. She must also actually consent to the marriage.
As for men marrying men, heterosexual men can't marry men either. So, the law seems completely fair on this issue.
Sorry, that line of reasoning was dismissed when interracial marriage rights were recognized in Loving v. Virginia in 1967.
To make a rational argument against same-sex marriage, you need to demonstrate a narrowly-defined compelling state interest which justifies withholding the fundamental right of marriage from same-sex couples.
Good luck!