This is not an instinct that's going to go away. It seems wrong to condemn them to hell for something where they "could not have done otherwise"

Where there is faith, there is a way.

There's nothing wrong with eating meat.

Since we're making appeals, I'll make an appeal to consequences w.r.t. masturbation. Even if it can be difficult to understand how something, as said earlier, is seemingly trivial or simplistic, can actually be something very grave, we need only look at the effect it has on those who habitually engage in such activity to get an idea about just how harmful this seemingly innocuous activity may be.

Men who masturbate lose motivation. This is a serious issue. Related to this is the emasculation of men who masturbate. Men who masturbate are also less confident, and far less likely to go out on a limb and take the risk of approaching a woman. Men who masturbate have weaker relationships (sexually or otherwise) with their wives. This weakens the family itself. The family is the cornerstone of civilization. What correlation might there be between endemic masturbation habits, and infidelity and divorce rates?

Women who masturbate, contrariwise, become more masculine. Instead of embracing a feminine behaviour, being submissive or obedient to men, they feel empowered - they "don't need a man" to give them such intimate pleasure. They are far less likely to marry, and even less likely to stay married if they do marry, if they have masturbated habitually or were otherwise sexually promiscuous. Again, this is damning to the family and civilization by extension.

Aristotle, and Aquinas with him, affirmed that nothing in nature is vain. Following from this insight (also present in Scripture), we can understand how those trees that bear evil fruit are themselves evil. The evil fruits of masturbation are, as also said earlier, too numerous and perverse to fully enumerate. It will destroy any person who engages in it habitually, and they will go to their grave ruining their best attributes and believing themselves to be doing something good or wholesome. Such an attachment to self-provided pleasure also lends itself to the making of pleasure into an idol - it lends itself to hedonism, where pleasure is pursued as an end in itself. Anything but God (and man, made in God's image) cannot be an end in itself, and even man is only so in relation to God. To view pleasure in this way is thus damning.

Repent and believe the Gospel - for your own sake.

Maybe not every moral truth can be exhausted by pure reasoning alone. The Christian always has the appeal to revelation as an epistemic modality for believing a particular moral truth.

There are a lot of fine details though too. For example, I don't consider masturbating to mental images to be equivalent to masturbation to pornography.

Many times, I take moral teaching in Christianity to have an esoteric and an exoteric component. On the exoteric side, we can construe the wrongness of masturbation as something having purely to do with just ends, or whatever.

On the esoteric side, I often find (personally, anyway) that there are likely natural explanations, which if we were to dig deeply into the full psychological/physiological effects of masturbation, would prove to have detrimental consequences.

In other words, morals like this one may say that the action is wrong because it is sexually nonproductive, but I also think that this level of reasoning betrays something more straightforward yet concealed - that Christian morals tend to optimize a person's psyche and experience of life (whether that be from physiological, psychological and/or social effects).

For example, masturbation may cause you harms that you (a) are not aware of, or (b) which you are aware of but which you don't connect to masturbation conceptually. We might say these are downstream effects which the sciences have not yet (or refuse to) connected to masturbation.

Perhaps these arise from the availability of masturbation and the convenience. That retaining semen and the effects this has on libido do something by way of conscious experience that has ripple effects that spread across your life. Or chronic masturbation effects neurotransmitter levels in the brain that alter your day-to-day consciousness.

I genuinely believe that Christian virtues identify a way of being that tends to strike a balance across multiple dimensions of life (and desires) that achieves something like a dynamic stability or optimization.

Perhaps masturbation once per week is more moral than 35 times per week.

Or that entering into a relationship with a woman and having sex with her is more moral than either of these.

Morals have ends, both for this life and the afterlife. I believe that.