If you say to a woman "I wouldn't f*ck you" and expect that to be a put-down, you're basically saying two things: firstly "I have a very high opinon of my own sexual allure, and I believe women will be upset or insulted when I say I don't want to have sex with them," and secondly "This is an insult because women are only worth judging on their sexual appeal, because all they're good for is sex."
For once Liz I'm going to have to say I disagree with you and I think that's over-analysed but my heads feeling too mushy to argue the point coherently, and certainly not with someone who actually knows what they're talking about.
EDIT: though I take Wow's point if you replace the sexism with racism, so maybe I do agree. Damn this fecking cold!
It is overanalysed, and yes, if you call someone an unshaggable old mare, you're probably not analysing what's behind the sentiments any more than "she's pissed me off."
But the implication that a woman's worth is in her sexual appeal is behind the concept that the most hurtful thing you can say to her is an insult directed at her sexual appeal / weight / likelihood that she's the town bicycle.
If you take away that implication, it's no more hurtful than a five-year-old saying "You're not coming to my birthday party." The force behind it is inextricably linked to an assumption that she, like you, measures her value in sexual appeal.