there are tons of gay porn vids based on that premise, but in every one i've seen i have a strongly suspect that it isnt' the dude's first time takin cock. and the acting is always bad, so even though he resists, he never really "sells" it.
even amateur sites like chaos men and sean cody etc, they always label dudes as str8 but u can tell they're not. ur more likely to find something real on xtube by total amateurs, but i haven't. I doubt if a scene like that really happened, the str8 dude wouldn't exactly want it advertised.