No, sex an O isnt selfish, half planet would be sick than, not only "3" man on earth, the logic and comon sense.
Ask eny of your frend, did they get sick after sex or mastubation, and evan overmasturbation. The answer will be NO!
How much sex and O is enoug for some idividua? Nature made that perfect, when is enoug for you , you will know it, its proces who end by it self.
Of course, if some one "torture" him self , that will not be good for that person probbly.