I agree, highly unlikely this is an STD. Not sure what this could be tho - you said you don’t masturbate all that much & don’t do it vigorously, so it’s a bit of a mystery. Do you have any symptoms at otherwise, like during or after urination? I’m guessing not, since you didn’t mention it.
I guess the best thing you could do would be to make an app’t w/ a Urologist- he or she would prbly want to examine you physically & observe the redness & any other signs of irritation.
Good luck with this issue & let us know if you find a solution...