I guess you're right that M is a way to get rid of sexual tension, but that doesn't stop it from being a sin. Theft is a way to get rid of envy - if you want something, why deny yourself it? - but nobody's suggesting that's not sinful. If a person's spirit is at one with God they have no need for such sins because they feel only love, not selfish emotions like lust or greed.

And yes, I agree with you that sexual tension is painful if you hold it in. Due to bad problems with our parents my husband and I had to share a home together before we married (we didn't do anything sinful) and there were times I wanted to touch myself where I shouldn't or harm myself because the frustration was so bad, but I thought of how once we could marry we would be able to do certain things with each other as often as we wanted and how God willing we could be with each other for many decades and how nice it would be to come to our marriage pure, and that stopped me.