I can't stop him once he starts

You're friend is wrong on both counts. If you tell him to stop halfway through, even after agreeing to have sex with him in the first place, then he has to stop. Otherwise it is rape, he has raped you. And I mean that, I'm not just exaggerating to prove a point. I'd never want to water down the word 'rape', but here it is perfectly appropriate. Plus it doesn't hurt our testicles to stop, maybe it'd be a bit disappointing, but not so disppointing that it'd make any normal decent guy want to rape someone.