Because when you do selfish acts, and that includes selfish sexual acts, you make yourself depressed in the long run. That's why selfish sex is considered a sin in the bible. It's more then just a moral thing. I've been there, I've been selfish before on that and it always took me down a path of misery. Sex is not something to be played about with. You people think that something that is designed to create a life is something you can just mess around with. Do you see animals having constant sex or being constantly obsessed with sex like humans in ( especially western society) are?

The reason why gay sex is considered sin in the bible is because two men cannot reproduce and nor can two women. Sin means " to miss the mark". Two men/ two women who have sex cannot reproduce therefore it is missing the mark. Also the vagina and the penis both have nutrients that a man and women both give to each other when they have sex without a condom. A man having sex with another man cannot do that. I suppose they could both swallow each other's semen but it's not the same thing.

A man who constantly ejaculates and masturbates is depleting his life source, is risking mental and physical illness. This is not a myth, a man gives away a lot of minerals when he ejaculates. But the doctors don't want men to know that and encourage masturbation so you become depressed and they can sell you their useless and dangerous antidepressants.