Ultimately this boils down to a heightened cultural sense that women are more vulnerable and insecure than men - a man who rejected a woman this way would be mocked or heavily criticized, under this standard, because men are supposed to learn to stoically accept rejection, and in addition are thought to be stronger, making the act of throwing coffee seem abusive rather than playful.

A woman is often seen as vulnerable. Thus, she uses her wiles to attract a man by appealing not just to his head and heart (presumably, through her personality), but also through his stomach. As the vulnerable party, she's putting herself out on a limb, and to reject her advance is to damage her while she is exposed. The suggestion of throwing coffee seems more playful and even a little more just, because the woman is not in the position of power, but she gets the upper hand, and the powerful man gets humiliated.

Of course, if men and women are treated equally in society, this should all be a bunch of bullshit. But I get the impression that much of gender dynamics in East Asia is still hung on some old gender roles. Even crueler, I've seen a lot of examples of very emasculated men in places like Japan, Hong Kong, China, who clearly are not the dominant force in any relationship, and for which an act like this would be more insult and abuse than cheeky playful fun.

Also, it's a joke, so 2nd degree burns caused by 'piping hot coffee' aren't really part of the intended image presented by this street-ad.

EDIT: I missed the part where this could also be some feminine-empowering bent, where the image of abuse is intentional. I imagine there are parts of the world where this is a popular idea. The same way that wife-beating jokes may have seemed funny here half a century ago.