In my experience, the classic answer in Christian circles for why masturbation is wrong is because it involves objectifying another person.
Makes sense. And because the teaching that it's wrong is usually definitive, there's usually no motivation for thinking it through any further. Classic sheep-think, I guess.
But then I noticed that choosing (or feeling compelled) to masturbate could be stress-induced. Taking it even further, I also began to feel like there could be a distinction between being horny/desiring intimacy, but that it didn't necessarily have to be projected onto someone else (though that's the obvious next step). I guess it was leading towards the question of, "Is it possible to masturbate without mentally objectifying someone?"
Even though I think that it's a heavily caveat-ed yes, I never realized until today that for the same reason (of many) that I believe contraception to be wrong - that the action of ejaculation should always be associated with the possibility of life (cf. Onan), it also applies to masturbation (usually cf-ed with Onan, but poorly). I guess it comes from the fragmentation of mostly being taught about masturbation from the Protestant church and about contraception from the Catholic Church. I did always feel funny shooting my swimmers directly into a toilet bowl (or mis-dripping them onto the seat; you know what I mean).
And don't get me started on pre-cum or separating the sensation of orgasm from the action of ejaculation. Yay for rabbit holes. Or capital-m-Mysteries.