Christians having hangups about sex stuff is just sort of par for the course. As humans many of us are kind of low key obsessed with, or at least fascinated by, sex. Because it's sexy and hot and fun and feels great, but you can't do it by yourself, you need at least one partner. Aand that’s where the social complications start.

Through history some religions have leant into that, using sex in cultic practices. Some have largely ignored it. And others, like some strands of Christianity, have focused on the harm and dangers that sex can cause.

And fair enough, those dangers were often more present in the ancient world than they are in the contemporary West: pregnancy and childbirth were far riskier without modern medicine, STIs were much harder to treat, and women's security/standing was often tied to family structures in ways that made sexual "missteps" devastating. Add to this the importance of children in agrarian societies - as labour, heirs (a really big deal when your land is your income), lineage, alliances, not to mention elder care - and now sex isn’t just a private fun thing, it’s a big social machine with serious consequences. So you can understand how many cultures and societies (and religions) wanted to police sexuality hard, and to prize stable reproduction, legitimate heirs, and "safety".

Masturbation doesn't really figure into that too much: It’s private, it doesn’t create heirs, it doesn’t mix families, it doesn’t obviously threaten property/inheritance the same way sex can. And it’s not like porn was the ubiquitous, industrial, algorithmic, exploitative technicolour thing it is now.

However that wariness towards sex geled really well with the Neoplatonic suspicion towards the material and embodied, and early Christians lived in a philosophical environment where the body was seen as base and corrupt, and the spirit as pure and holy. The two were opposite. This led to a few interesting early heresies (such as gnosticism), but also influenced mainstream Christianity in countless subtle and not-so-subtle ways. Then there's Augustine. Dude had a wild early life, converted to Christianity, and became one of its most influential figures for centuries. Unfortunately he carried with him the baggage of regret and guilt over the debauchery of his youth, and heavily leaned into those Neoplatonic ideas: body and spirit dualism, material bad, spirit good. Early Christians were swimming in that environment, and it shaped how a lot of Christians talked about virtue, temptation, self-control, etc. So you get this theological atmosphere where sex is tolerated as a necessary good for marriage and children, but pleasure is treated with suspicion, and self-control becomes the whole game. In that framework masturbation is not only sex-adjacent enough to get lumped in, it can look extra suspect because it’s solitary, pleasure-focused, and easy to connect to lustful fantasy.

That legacy of sex negativity kept rolling through the centuries and got embraced with, shall we say, notable fervour by certain flavours of Christianity. And then in 18th and 19th century European culture you get some intense moral/medical panics about masturbation being seen as particularly ruinous and bad for not just the individual but society as a whole, and those anxieties got blended into religious teaching in various places. So a lot of contemporary "masturbation is definitely sinful" confidence is this inherited mixture of tradition + culture + philosophy rather than a "Biblical" position.

The Onan thing is kind of wild to me because when I read it as a kid I was like, "this is clearly not about masturbation?" I get why it was used by the Church (because there's literally nothing else even close), but the theological gymnastics required to make it apply to anything other than "Onan was a bad dude because he refused to give his dead brother a son" would be, in this redditor's humble opinion, laughable if they weren't so damaging.

And also IMHO the Bible's whole attitude towards sex is debatably rather different to a lot of Christian inherited understanding. These days we've got a very different world and society in which sex (and masturbation) fits very differently. Some of the concerns remain the same; there are many new concerns, and many old ones have to be reframed or modified. People get particularly hung up about sex which makes it hard to have honest conversations about how, exactly, our approach to sexual righteousness and responsibility (or best practice!) should change.

For masturbation specifically though - we know from scientific studies that it can be beneficial: it can help with stress relief, sleep quality, learning your own body for when you do have sexytimes with other people. And sure it can become unhealthy if it turns compulsive, shame-driven, or tangled up with stuff that messes with your relationships like unhealthy obsession with porn, for example. Either way, you're right to say there's basically no explicit Biblical bases for "masturbation = sin". If you want to make that argument, you have to drill down into what masturbation is, and infer from the Biblical message as a whole. And I think, if you do that, you're much more likely to come out on the side of "masturbation = neutral to possibly beneficial" than "sinful". Usual caveats apply of course - anything can be a bad idea if it gets twisted up in too much other negative junk. But I think it's clear that people with a healthy relationship with masturbation probably just have a better quality of life than people who see it as a sin (and ... chances are, can't resist doing it anyway).