Is masturbation a sin?

It depends on what you mean by "sin". Sin is a religious concept, and the definition of what is a sin depends on what religion you are accepting as True.

I was once a Protestant and I know that some American Protestant denominations believe that masturbation is a form of Lust, which they feel is prohibited by the Bible (for example Matt 5:27).

That was long ago, I now see masturbation as a natural part of a normal sex life.

Everything that's not virtuous is sinful, and there are sinful aspects of even virtuous things. This is why forgiveness/salvation/redemption/sanctification are not simple, quick processes. Once we accept Jesus, we have only initiated the long path of sanctification, i.e. cleansing from sin.

Why is masturbation sinful? The same reason why other non-reproductive sex is sinful, i.e. because the natural purpose of sex is procreation. St. Paul discusses marriage, abstinence, virginity, etc. in 1 Corinthians. He doesn't discuss masturbation specifically, but I think you can find references to 'impurity' in the bible. Sexuality is a slippery slope. Any form of sexual indulgence stokes desire. It is mostly impossible to remain completely clean physically and mentally and refrain from any and all sexual experience. It just happens and that's why we need to be honest about it in prayer and accept God's forgiveness and maintain faith in salvation and sanctification. As time passes, we grow in self-control in all things, including sexuality . . . if we remain committed and don't fall to the temptation to give in to hedonism.

Compared to other systems of philosophy Hedonism makes a lot of sense.

A Hedonist acts in such a way to maximize the amount of pleasure in others, and in himself. To me, pleasure means happiness over the long term. I am willing to put off things that might me happy now in order to reach long term happiness for myself and for the people I care about.

Masturbation is pleasurable, and sometimes it is something you can share, and it doesn't hurt anyone. Under a Hedonist philosophy, it is desirable.

There are not very many Christians who are against non-reproductive sex. Most Christian denominations allow for sex within marriage that doesn't lead to offspring.

A life without sex would be pretty miserable for me... and I am done having children.

Hedonism actually undermines happiness. Think of yourself or any other organism as a pleasure machine where the potential for pleasure builds up within the organism/nervous system. The organism is evolutionary designed to perform all these functions for the benefit of its own survival, its family, species, ecosystem, planet, etc. The forms of pleasure it experiences have evolved to connect it with these various beneficial functions, such as reproduction, nourishing itself, caring for loved ones, exercising, etc. etc.

Now, the human brain evolves to be extremely clever and starts finding shortcuts to achieve the same natural pleasures that benefit our survival, others', the ecology, the planet, etc. E.g. we figure out that we can refine sugar, which tastes sweet because we need energy, but now we can overeat sugar and harm ourselves with it. We also figure out we can harvest and burn fuels in combustion engines and run machines and thus save our own body energy, so we end up getting less exercise and overeating while wasting industrial energy, depleting planetary reserves, undermining natural ecological resource cycles, etc.

So hedonism in general causes this seeking of shortcuts to pleasure that harms us. So a better philosophy is one that recognizes that pleasure comes naturally as a result of performing productive functions, so choose duty and productive work ethic instead of hedonism and realize that deferred pleasure is actually more pleasurable anyway, because the potential for pleasure builds up in our nervous systems by resisting immediate gratification, and then we end up experiencing more pleasure in the long run for having developed the discipline to defer.

St. Paul in Corinthians talks about people who can't resist lust and encourages them to marry because it is better to marry than to burn in passion. In other words, it's good to resist sexual desire and remain abstinent and virginal if possible, but if not you should contain it within marriage, at least.

Read 1 Corinthians. It is very clear and straight-forward writing.

I'm just glad we seed our lava go onan on about it in his usual fashion.

When you have nothing to say, simply ridicule the people you disagree with to demonstrate your stunningly clever capacity for sarcasm, eh?

I find that having sex is quite pleasurable, even though I am done having kids, is quite pleasurable. I am not ever going to have more kids. So, your "potential for pleasure" argument doesn't make any sense.

But I am really writing this for the original poster.

Sexual energy is just a manifestation of life energy more generally. When you rechannel that energy away from sexuality into higher callings, sexual pleasure is redistributed to the nerves stimulated by those other pursuits. I.e. there is a shift of pleasure-release within the distribution of the pleasure reaction within the brain.

It's the same when you avoid sugar or spicy flavors and your palette becomes more sensitive to them and the food you do eat tastes good. In fact, the hungrier you get the better the food tastes when you do eat, and when you aren't focused on food, other things you are doing fulfill and satisfy you in a way that's similar to consuming a meal.