Does the Consitution's "equal protection" clause imply that siblings can marry?
That's an interesting one. So far, no, because of the extraordinary risk to potential children, plus the cultural taboo of incest. Over long times, taboos change though.
Does is imply polygamy and polyandry?
It might eventually, provided that that you can properly document the consent of all parties, and that no one is getting abused in the process. There is also a cultural component to it.
How about rape, since I have a sexual urges to many women who aren't interested in me, but married people have conjugal rights?
That is clearly violating someone else's basic rights.
If I can find a willing mare in heat, do I we have an equal-protection right to marriage?
I don't think you can document the mare's consent, plus, it (she?) can probably not fulfill a number of legal responsibilities. Basically, it's not much of a citizen.
You can address all these questions. Some are trivial, some are much more subtle. But the ones that are actually at stake in the political debate these days are mostly on the simpler side of the spectrum.
When someone pushes a new law about polygamy, I expect to read some interesting arguments. The reason it's not going to happen soon is that there is very little interest for it in society, so it stays mercifully out of public debate.