Here's the litmus test that will never fail. When you choose your spouse, you understand that if it were ever necessary, you would willingly sacrifice your desires for that person. I don't mean trivial silliness, I mean when the chips are down you do what you need to do, realizing you made a COMMITMENT and you abide by it.
People unable to go there definitely have no business getting serious with a partner.
I don't know... Depends on what you mean by 'serious'.
I say, as long as everyone is honest and on the same page with regards to where the relationship is at,