I don’t think her refusing to be exclusive until engaged makes her a c*nt.... Fact you want to call someone names for that choice, I think really speaks more to your character than hers.
I really don’t see a “trick” here, if a man truly wanted to be exclusive with her for the rest of their lives, then he really shouldn’t have a problem asking her to marry.
I don’t think that’s the best way to start a healthy relationship, but its hardly a “trick”. After all, what reason should a man or woman have to be exclusive, just because the other one desires it, but offers no benefit for it?
Pretending you are pregnant is a trick... being completely honest and up front that you have no intention of being exclusive until engaged is not a trick.
It does make you manipulate though. And she’ll do that throughout tree marriage.
I think the better approach would be to be "exclusive" for at least a few months before playing the "we're not going to be exclusive unless you commit" card. The problem with that is that really would be an ultimatum that would make the guy feel pressured. So that's a downside.
Her idea to state right at the outset that there will be no exclusivity -- except for sex - at least has the advantage of honesty and directness. And it's actually kind of interesting in that it really raises the question of "do I want to lose this girl to someone else" for the guy. And that's a perfectly legitimate question for a guy to ask himself.