As for the OP's 'problem' I think it can happen for a bunch of reasons of course.

Just a suggestion, but anytime you 'can't' fuck her, just tell her with confidence and authority that you "won't". There's a BIG difference between 'won't' and 'can't'. Won't is sexy, can't isn't.

Make her wait for it and get worked up, build up the anticipation, that shit's as good as a full blown pounding to most girls anyway.

Get her off with your fingers/mouth just by talking to her or whatever, then leave her hanging and wanting more untill YOU are 'ready' to give her more. Make her slow down and be a 'good girl', it'll get her frustated in a good way and buy you the time to get going and catch up with her.

If you let it get to the point where she's sucking on Mr floppy regularly, then she might start have doubts about how attracted you are to her and those doubts can get harder and harder to deal with the longer they go on unchecked.

Personally I think it's (probably) just about taking control over the situation, so she's not in a position to spring sex on you at random times in ways that throw you off guard.

That's my 2 cents anyways for what they are worth.