Maybe. But it’s still a different case from “let’s get married forever, and break it off if it somehow doesn’t work for some reason.”
Contracted cohabitation would be a useful option for, say, the college years or early career years, and would dispense with the pointless emotional wrangling and legal battles that burden even the simplest mutually-desired divorce.
I guess you could make an argument that it could be more likely, because you can’t dissolve a same-sex marriage on the basis of non-consumation, mainly because nobody could decide what exactly consumating a same-sex marriage would entail.
But it would be a pretty crap argument, given that you could use similar evidence to non-consumation to get a divorce.
No. You can’t marry your sibling, gay, straight or whatever.
Don’t you have to have a wedding to get married? Standing up in front of some official and saying the required words is still a wedding even if you have no guests.