Catching her poking holes in my condoms. Assuming that I actually love the person and we'd be going out seriously to start with (wouldn't go out with most personalities, never with an overweight person, etc) this is pretty much the only thing. I could probably put up with most things, like cheating (not really a big believer in monogamy, and I've had sex with girls despite loving another one), drugs (I've used/tried them a bit myself, if someone I loved had issues with it I'd help her get out of it instead, smoking inside would be a pain though), religion (well, if they can't handle me arguing about it they'd probably dump me though, I love arguing:P) and 'bad' humour (but if they don't like black humor we probably wouldn't start going out in the first place).
Getting a kid right now would ruin my life though, I couldn't handle not travelling and I'm currently a student (I'd find not using protection a far bigger 'betrayal' than cheating on me).