Woah, didn't expect so many damn replies. Thanks for all the input though... though I do hasten to add to not entirely judge my character on this one scenario where I'm willing to admit that I fucked up on a few things myself.
First, I didn't really stop the sex to have a discussion as such. It was moreso that she just had a weird, negative reaction to the fact that I'd suggested oral sex and it seemed like something was wrong with her otherwise, so the words "What's wrong?" just kind of slipped out. Then things more or less just escalated from there, entirely killing the mood. I didn't immediately demand an extensive explanation, I just sensed there was something "off" with her mood.
Yes, I've apologised and we seem to be fine now. She's going to try and communicate her needs better to me (which doesn't mean mid-sex debate and discussion) and I'm taking a concious effort to be more understanding of her feelings. And yes, there may be some other relationship issues that may have had an influence and the alcohol and marijuana definitely didn't help the situation.
My defence is that I'm in my early twenties and I'm quite often a fucking idiot. I still think I'm an awful, horrible bastard - but now at least it's for reasons other than the failed Friday night sex. Oh well!
Cheers for all the input, though. It's been genuinely helpful to get a few perspectives on this.