Well, some folks might question whether is belongs in the realm of masturbation, but I think it absolutely qualifies. Anytime you are sexually stimulating yourself is a form of masturbation. Yes, all you straight guys, even giving it a few strokes to harden it up enough to slide in,
or stroking to cum on her tits (or face)
is jacking off.
None-the-less, it sure feels good, right?
In my younger days, I was quite capable. One military buddy told me if he could do it, he'd never leave the house! Was he jealous? Probably. But it takes some work to get limber enough. So, if you can manage this, enjoy it for all it's worth. You've earned it.