Same-sex marriage isn’t anywhere near the bottom of the slope, not unless you’re one of those really weird fundies who thinks same-sex relationships are morally little better than murder. Even if you’re one of those people who thinks all of the possibilities are equal, there’s still a “slope” in the sense that adoption of one could (theoretically) lead to the adoption of others. Typically, the slope looks more like this:
0. Only traditional marriage according to the dictates of my religion.
1. Secular marriage is allowed.
2. Mixed-race marriage is allowed.
3. Same-sex marriage is allowed.
4. Polyamorous marriage allowed, which is “bad” because it would allow a woman to marry two or more men. Or a small number of men might hog all the women, either way.
5. Incestuous marriage is allowed.
6. Pet and/or livestock marriage is allowed, and you know that some of them are going to be zoophiles.
7. Object marriage is allowed, rendering the whole concept meaningless.
8. Pedophiles manipulating children into marrying them is allowed, because ha ha what even the fuck matters anymore.
There’s some flexibility to the order of 6-8, as which of these seems most likely to happen first depends on one’s philosophical inclinations. Regardless, most conservatives want to dial things back to somewhere from 0-2, most liberals have long wanted to reach 3 or 4, 5 is a very much minority position, almost nobody wants 6-8, and the latter is not likely to ever change. There’s a real slope, but it doesn’t go all the way to the bottom.