I have to think it over, but that madonna-whore thing sounds damned spot on (with me, at least). Anyway:
>And more interesting, if someone is against it: why?
Disregard its reproductive function (which I dislike as well), together with its dangers and problematic situation as an instrument for violence, manipulation and control, my main problems with sex are:
1) It is endlessly disappointing. Sexual arousal and orgasm never live up to my expectations of passionate erotism, let alone as a materialization or proof of love. So I find much more excitement in the longing than in the fulfilment: I prefer those sensations and feelings to remain as mere potencies, within the realm of unbounded possibilities, than to land and crash in the condition of mundane matters of fact. It might be just my own poor outcomes but, in any case, the most erotic moments I've ever experienced had been with no actual touching or self-touching involved, in mere solitary contemplation of some piece of art; which somewhat leads me into...
2) It is counterproductive, insofar as purported "reaching" of the other. This would need of a broader explanation involving some semiotical and ontological considerations. However: I believe that touching, with sex as its epitome, rather that uniting people irreparably separates them, as physical contact constitutes the verification of bodies' existence as different entities; which will never allow actual people to really "become one", as my functional definition of love demands. It is not a conclusion I'm happy about, not at all; but it's something I cannot overlook and do not think I should.
Thus, mental visualization of fictional characters, incorporeal and non existent, it's the only way I can acceptably experience not just love but erotic pleasure as well. Except for a couple of very weak exceptions, masturbation has nothing to do with this and it's rather considered like the handling of a periodical itch or physiological function (like needing to shit). Having actual sex with someone is, hereinafter, simply out of the question.