I am perfectly happy with all of my shoes that go great with all of my outfits. But, I will never pass up an opportunity to buy another pair of shoes!! This does not mean that I will not continue to enjoy and love wearing all of my other shoes on a regular basis.
My point is, although you both are very happy with your sex life, he just so happens to enjoy the entra sexual relief he gives himself. That does not mean he is unsatisfied with you in any way, shape or form. What he does in the shower with his own body has nothing to do with you. He masturbates because it feels good to HIM.
I am a foodie and I love to go out to eat with my spouse, but sometimes I just like the solitude of dining out alone. This does not mean I like food more when I dine out by myself and I still look forward to dining out with my spouse, just the same.