Trigger warning-- I plan to address the topic of the OP. The Bible addresses sexual issues, so I consider it as an area that may be addressed by Christians.
I don't see where the Bible teaches, 'Thou shalt not masturbate.' I am not going to tell people to do it, but I'm not going to tell some young person that if they feel like they are going to explode down below and do that to alleviate physical pressure or whatever and pent up hormonal frustration that they are sinning. We need to keep a good conscience toward God.
Now, in modern Greek, the word for 'malakos' in I Corinthians 6 is or includes masturbation, but I do not think that was the case back in the first century. It could be one of the things malakos did is use their hands on other men, so that's how it shifted in meaning. I don't know.
In the Old Testament, it mentions a man in the camp having an emission and being unclean and washing his whole body with water, as I recall. But any man who had sex with his wife and 'finished' would also need to go through the same ritual. If he didn't complete, I suppose he would still be clean. Being unclean and sinning weren't exactly the same thing. Israelites were supposed to reproduce. It's presented as a blessing. Having sons and daughters was commanded to those going into captivity in Babylon-- increase and do not decrease. Married couples who had sex were unclean until evening. I would imagine that would have encourage sexual activity to occur a hour or two before sundown, so they could be unclean for a short period of time, but I don't know of any evidence that Hebrew strategically timed it that way. That would probably be prime working time during the planting and harvesting seasons.
Anyway, masturbation would have made Israelite men unclean if they finished the act until they washed and the sun set down. I don't there are any uncleanness restrictions when it comes to female release. Orthodox Judaism encourages men finishing in an act of intercourse and some interpretations are rather non-restrictive otherwise. There were voices in Roman Catholicism forbidding all kinds of sexual acts, even intercourse in positions beside missionary, but I don't know if those were ever considered official teaching. Augustine was rather influential, and I think he was opposed to female superior position, probably based on an etymological fallacy, that that puts a woman in some kind of superior position. He was one of the celibates who actually had had some sexual experience with a mistress or concubine during his BC days.
The Bible doesn't say 'spilling seed' is forbidden or sinful. It was for Onan, but there was a specific issue of his not wanting to raise up seed to his brother. The idea that the sin was 'wasting seed' seems a bit of a stretch, though that seems to be the reasoning of Roman Catholicism, maybe streams of Orthodox Judaism also. Proverbs warns about men spilling waters in the street, but there is the context there of not having sex with another man's wife.
Another line of thought is that if masturbation is a steam pressure release valve for lusts, physical urges, physical pressure, etc., then why doesn't Paul suggest it in I Corinthians 7, instead of just offering marriage as a solution for preventing fornication? I suppose that is a question a young man might think of if he is worked up. But I wonder burning in 'It is better to marry than to burn' has to do with a lot more than physical urge for release that can be so intense among the young. There may be a burning to be loved, for that to be expressed physically, for the whole physical and psychological experience of marriage and sexuality, maybe the desire to have children, the desire for companionship, the desire to have children. All of these could contribute to the desire to fornicate. Still, Paul does not _recommend_ masturbation.
It can also be difficult for a young married man, physically, to stop having sex during that one week out of the month if he is used to very frequent amorous activity with his wife, and he gets used to that level of frequency, so I can understand the concerns of the OP. I since Hebrew men were unclean if they touched their wives or their wives touched them, certain things were off the table. I don't think Gentiles are that restricted. Gentiles were driven out of the land for uncovering their wives nakedness. A man can keep his wife's skirt on, and not uncover her nakedness. I don't see spilling seed as a restriction in scripture, if a wife wants to be generous in other ways if a man is tempted to handle his predicament himself.
But if it's an issue of a wife or husband touching one another by hand, or even handling oneself before or during the act as part of the overall experience of intercourse or leading up to it, or something like that, I don't see where we have a leg to stand on condemning that. Foreplay can help prevent damage and pain.