And accusing someone of 'mental masturbation' is on par with daydreaming or wild speculation.
In common parlance, it has a rather more negative connotation - masturbation is something done specifically for personal sexual pleasure, is publicly considered to be slightly dirty, that should be done in private, if at all. There is an implication of purpose and intent - sexualized self-pleasuring - that is not really present in daydreaming or wild speculation.
As an analogy: How would you like it if someone said you were threadcrapping here to stroke your own ego - because you found it easier to make yourself look good by tearing other people's things down than by building something yourself? I doubt you'd like the insinuation. Now, apply the Golden Rule, and don't refer to other people's thoughts as masturbatory, or in other disrespectful forms. Please and thank you.