I think it is a bit humiliating to have your genitals grabbed and be forced into cumming by somebody stronger than you, as an expression of their power over you. It's probably the most complete defeat that one person can suffer at the hands (no pun intended) of another, and goes way beyond a simple wrestling submission. She's now had you in a one-sided manner, and you both know it.
The slight sense of embarassment/humiliation kicks in after the sugar rush of ejaculation, as you survey the sticky mess she's made of you and realise the casual authority she exerted over your body just for her amusement. I know this is overthinking it, but it's like your erect phallus goes from being a symbol of male power to a weakness, something she can use against you to demonstrate how helpless you are.
Obviously it is all consensual and the thrill is that a woman is able do that to you, that her strong hands can go wherever they please and do whatever they want.
I think being jerked off while sitting on a woman's lap also feels quite humiliating and transgressive, whether forced or not - and it has been done by force to me.
Being fingered is probably another step beyond that but hasn't happened to me, despite half joking threats by a few woman who could have taken me by force if they'd really wanted to.
And being fingered while being carried in her arms is almost a double humiliation.
Cumming inside her never feels humiliating, even if you have been completely overpowered. It's still mutual sex and you're both having each other.