**** Married Geezer Alert ****
First off I want to congratulate you on your decision to stop fornicating. I am sure it will be a difficult one but hang in there. God's grace is good.
As to whether "M" is a sin or not has been hotly debated for the last 40 years. Prior to that it was pretty much assumed that it was, going back to the early 1700s. Before that, it seems it was not much thought about one way or the other.
It is interesting to note that the OT purity laws list a whole host of aberent sexual behavior that is forbidden. Not one mention of "M." That is especially noteworthy because it is depicted in the heiroglyphs in Egypt from the time they were there and it played into Egyptian worship practices.
According to Jewish tradition, part of the abuse the Israelites suffered from the Egyptians was the forced seperation of husbands and wives (no sexual outlet). From the Traditional Passover Haggadah:
"And He saw our affliction" – This refers to forced separation of couples. As it said, ―God saw the children of Israel and God knew.‖ Exodus 2:24
It may have been that it was left out so people could have some badly needed relief, either by Moses or by God Himself.
Whether that is the real case or not, sexual frustration was not taken lightly by the anceint Jews (especially female frustration) and they intrepreted the scriptures to say that sex was a wife's right and a husbands responsibility. (reverse of western opinion)
In my opinion, what makes it sinful is 2 things - if you involve another person and where your mind is at. Sex binds us together at an emotional and spiritual level and if someone else is there you are binding your self to them apart from a marriage covenant. If you fantasize about having sex with someone while doing it you are lusting and coveting.
So if you can avoid both those, you are probably OK unless you feel convicted by God to stop. (but don't confuse the emptyness in the pit of your stomach of not having someone to connect to with guilt or condemnation - they are not the same thing)