Thoughts!
If you are in a committed relationship is watching and/ or pleasuring yourself to porn cheating?
It depends on the definition of "cheating" a couple has chosen.
In gay relationships, there's often a healthier attitude about individual sexual openness. That includes open to sex with other people, open to solo masturbation, open to solo play with toys, open to porn, etc. But it only works when both persons in the relationship agree on the rules.
Opposite sex couples have a way to go on the topic. If it's something that is causing a problem in your relationship, have the discussion instead of making porn the problem.
If you visit the Straight forum, you'll see similar discussions about pegging and anal sex. In opposite sex couples where the man enjoys being penetrated anally, the couple might try pegging, or they might invite an additional male partner to join them occasionally, or the wife might agree to let the husband pursue relationships with men outside the marriage. The common thing is that it's been discussed and the couple has agreed on the rules.
The same thing would be true about watching porn. It's whatever the two of you decide is okay.
I have watched porn with my better half and by myself forever & I know my wife watches lesbian porn all the time. I personally do not think it's cheating.
The question is largely a matter of how much and why.
In the case of your wife, lesbian porn is something that she enjoys that she won't be able to find in her relationship with you. As long as the porn isn't a substitute for real sex with the committed partner, it's not "cheating", nor is it harmful.
On the other hand, if one of you is using porn as a substitute for a healthy sex life with a partner, the issue isn't cheating, the issue is the relationship. Substituting porn to make up for something lacking in the relationship is not the fault of the porn; the problem is the that there's something lacking in the relationship.