Ok, so here's a little flaw in your analogy - s/he decides that this isn't it/wants marriage or kids after you've determined that you didn't want them/wants an open relationship/etc. etc. and you're like, "No, that's not what you said in the beginning" then all of a sudden you realize that you've just jumped off of a fucking cliff and are plummeting to your death because you just trusted someone who wasn't who they said they were.
How about just being friends on flat, even ground and being nice and just going for a drink and see how things work? It's hard to enjoy a nice merlot whilst BASE jumping. *sheesh.*

You, uh... you can put those bags anywhere, Zack.

They're mine and you can't have 'em.