(A) The purportedly plain and simple sin is not mentioned so much as a single time in the Torah. Given that the act occurs almost infinitely more frequently than bestiality, and you and yours claim “grave” and/or “mortal” sin, one wonders why it isn’t mentioned at least the one time. Also more frequent than adultery, though adultery gets the nod on the mention [adultery makes the Top 10]. And given the 90/10 ratio of hetero to homo, seems that masturbation is more frequent than homosexuality as well, yet the latter and not the former gets the mention.
(B) Who defines you, you or your hormones? And so there was a time without the testosterone when the act didn’t concern you and then a time with testosterone when it did. And here I thought that it wasn’t with flesh and blood and biochemicals but instead the spiritual powers of darkness in the heavenly places that I am at war with.
(C) Selfish? What isn’t selfish? You getting to heaven for someone else and it’s all altruistic for you? So don’t be absurd. I’d rather he: (1) masturbate during the 7th inning stretch while watching the game on TV and (2) send the money saved on parking, the game ticket, and the inflated food and drink prices to a charitable organization keeping other humans from dying of outright starvation or chronic severe malnutrition, than he instead do: (3) the “unselfish” thing of skipping the masturbation, plunking down $15 for parking, $30 for the game ticket, and another $25 or so for food and drink, and if there’s no bread for some others, well, let 'em eat cake. So be careful here as well as some things might be more and less selfish than you conceive them to be.
(D) The biochemistry isn’t violence, since if it was, it would likely be avoided. So rethink your construct, as the violence notion is preposterous on its face.
(E) Lastly, and by the way, going back to item (C), even the reading of a good book might be perceived to be rather selfish when 3 mil little ones, age 5 and under, will die this year across the globe from preventable disease. And sorry for having to mention the 3 mil, age 5 or under, in a year, as now you can never say that you didn’t know. Oh, and at least the mastubator can say that if it wasn’t for the testosterone that he didn’t once have, but what can the reader of a good book say [that 3 mil, age 5 and under, in a year, isn’t tragic drama enough]? So, again, careful with the selfish.