Okay, more seriously, OP -
I think what bothers you is that you think of wanking as a private activity which belongs somewhere discreet. It sounds as if for you, your BF wanking in your bed is the equivalent of him peeing in a vase because the bathroom was busy and he couldn't be bothered to wait.
Whereas to most of us on this thread, it's probably more like...scratching an itch? I don't know. But definitely not the disrespectful equivalent of peeing inappropriately.
Would it be okay for him to do it while next to you? Depends. Honestly, if my DH asked me for sex and I'd turned him down, it wouldn't bother me. (In fact I might kind of enjoy it. ) On the other hand, if he left the bathroom door open while using the toilet, I'd be as disgusted as you are now.
I don't think you're a prude, I just think your attitude to physical privacy is different to your BF's. In the end it doesn't matter what we all think, what matters is that you and BF have different views on it.
But really, being the Wank Police about it isn't going to help. Perfectly fair to ask him to be discreet if you don't want to see it (although it sounds as if he was discreet, to be honest). But asking him to wait till he gets home...well, that's a bit much, isn't it? Would you have preferred him to go downstairs with a massive hard-on?