May as well commit the sin, then.
Good point. In order to masturbate, a person usually must imagine having sex with a specific person. That then is an act of lust and psychological adultery, unless the person whom you are thinking of is your spouse. IOW, a married person, who is away on a business trip, can "self-stimulate" when thinking about his/her spouse, but not when thinking about somebody other than his/her spouse. And of course a single person should do not it at all.
The Bible says that we are to think unselfish and pure thoughts. This kind of psychological intercourse/adultery only makes it easier for us to be tempted to do the real thing. Sex, like our other actions, starts in the mind. We are to "take all of our thoughts captive to the cause of Christ." (2 Corinthians?)
Personal story. Back in 1979-80, I dated a really cute and fun Christian gal for about 18 months. We had lots of fun, but I never even held her hand or kissed her, although we had lots of good hugs. I intentionally tried to help her grow in her faith. In the years since then, I have never even been tempted to think of her in a sexy way, even though I am single. It was just a good clean relationship in which I wanted to go good, even though at the time I had to battle my desires. The relationship was not about sex. Masturbation is about wanting to fulfill our own needs. Instead, people might try to think about a very satisfying time when they did something good to serve and fulfill another person's needs. Just a thought.