I believe that masturbation is not a safe activity, even if your spouse is not giving you intimacy. I am in the same boat. My wife does not get any pleasure from sex (never has), but she is willing to help me, knowing that I am trying to overcome my porn addiction. We only have sex once every week or two. But it's good to build self strength, and let the body naturally release it through dreams. I often ask myself...if I were single, would it be Ok to use porn and masturbate, just because I don't have a spouse? No...those guys have to be stronger than me, because they don't have the option of sex with a spouse. Hang in there!

Are you using porn or other images to masturbate? If so then it is a problem. You said an almost sexless marriage...so you are still intimate with your wife. Society has taught us that we NEED a sexual release but it isnt true what we need is intimacy. What will happen if you dont ejaculate for a few weeks, months? NOTHING ... Sex addiction therapists work with addicts that are single and after a 90 day reboot they have tips for healthy masturbation.

Until you address why your marriage is "almost sexless" it's difficult to know what role masturbation should or shouldn't play.

I've read and read about porn abuse, masturbation etc.
Porn tends to lead to addictions. (I'm there at the moment).
Most every religion speaks against masturbatiin. In ancient times people believed you only had so much sperm. So don't masturbate or you could run out.
Virtually every man in any religion masturbates. Study after study has shown this.
And anything can be bad if its to much/often.
In moderation, you should not have a problem. How often is that? Depends on you and your own desires/needs.

I have an issue with bringing religion into this issue. If you side with religion, it's bad. But if you side with medicine/science, it's normal. Even married men will masterbate. I believe it to be natural. So I do agree with Seamus ("Virtually every man in any religion masturbates. Study after study has shown this"). I do think moderation is good, for anything we do. Many therapists will even suggest videos/movies to help kickstart or assist in love making with your partner. Again, in moderation.....the key word.