Speaking as if this was me and my partner, uh, if he says I HAVE to help him, fuck no, I'm not giving into some sort of pressure just because he's horny, I'd never force him either. If he just ASKS for "help" of some sort nicely, if I'm up for it, sure! I just can't stand that sexual "pressure" when people say you HAVE to do something whether you're in the mood or not, that's not ok, regardless of how horny or pent up the other person is.
if it's a STRANGER? Man I'd about punch him, that's fuckn RUDE. No one EVER "HAS" to help you do shit, if that's your pickup line, gay or straight, man or woman or anyone inbetween, that's fucked up.