1) You could say that both groups are human beings.
....but so are individuals over 18 and individuals under 18. Yet they are not granted equal access to marriage. So that alone doesn't grant equal rights to marriage. Pedophiles fall into this category.
2) you could claim that both groups are equally consenting adults.
...but that wouldn't alone grant equal rights to marriage because first degree relatives are in this category and can't get married either.
3) you could claim that two homosexuals are in love like two heterosexuals would be.
..... Yet love alone can't grant equal rights to marriage. You and your (insert family member/pet/innate object here) may feel you are in love but cannot get married.
4) you can claim that two heterosexuals can create a loving family unit and raise children who need a home.
...yet again, polygamists and 1st degree relatives among others could fall into this category.
5) you could claim that homosexuality is equal to heterosexuality in that each is simply a sexual orientation one has the option to choose from.
...yet orientation is simply an affinity of one thing for another. Homosexuality is a behavior. If you were to infer that homosexuality is an orientation then you would also have to concede that other orientations are on equal ground with homosexuality based on the simple category of alternative orientation. This would include incest, beastiality, and pedophilia. All of which are sexual orientations that without accompanied behavior are legal.
So, it appears that one cannot claim a right to marriage by simply being two (or more) consenting human beings (even of adult age) of a certain orientation who are in love and feel they can raise a healthy family. ...unless you are willing to grant marriage rights to a host of other groups such as polygamists, first degree relatives, pedophiles, animal lovers (you know the kind I'm talking about ) etc.
This is why it's not a slippery slope.
Here's another premise. Heterosexuals are different than homosexuals on a basic concept. All people (aside from genetic malformations like hermaphroditism) are physically heterosexuals. That is they are either physiologically male or female with corresponding parts. These parts are physiologically intended to attract to and complement the parts of the opposite sex. This is the intended design of our biology in order to propagate our species. Heterosexual behavior is the term given to this congruence. Homosexual behavior is a deviation from this and is an opposite behavior than our physiological biology intended. Heterosexual behavior is a congruence with our biology and homosexual behavior is an incongruence with our biology.
If our biology afforded a pathway for homosexuality to create genetic offspring then society could consider these two things simple sexual alternatives. But they are not.
Heterosexuality is a different entity completely than homosexual behavior.
So I don't see how separate but equal is even an argument here as homosexuality is not equal with heterosexuality.
Therefore if heterosexuals create a social construct called marriage it is within their prerogative to define it as being between a man and a woman. (On the way that any group rationally discriminates it's commonalities for participation) Outside groups have no legitimate argument to force their way into it without also conceding the door to Pandora's box.
I'm looking for a real discussion on this subject.