had a similar problem once. finally got with a friend id had for years, but felt so much pressure that i only got a semi chub while we were hooking up/feeling around before actually starting.
she pulled out my 50% hard manhood , grabbed it, and said “oh, someones getting excited”…and for whatever reason, it all hit at once. i felt my asshole quiver, my balls squeezed, my abs tightened up, and i moaned loudly while cumming uncontrollably.
she played it off, and had me pleasure her after, but didnt talk to me for a while after. she brings it up today and laughs at me.