Your sister's standards are too high. Nobody can marry someone against their will. Nobody can marry more than one person, so that's not an equal protection issue. Should you be able to marry more than one person? Hey, why not start a forum about that subject?
Ever notice that when you tried to make an argument against gay marriage, you never actually make and argument against gay marriage? You bring up other subjects.
Here's the most basic one for you, explained without pictures:
No, actually they shouldn't. Nature has shown us that 2 of the same gender are not complimentary. You should know that. There are obvious differences between men and women. You should know that, too.
It takes 1 of each to reproduce. You should know that. The 14th Amendment granted full membership in the human race to negroes. You, of all people, should know that, as well.