Straight guy porn often refers to sex videos of two men with one being evidently gay-as shown by mannerisms and actions, and the other as straight and manly, obviously.
Especially gay viewers who jack off to this kind of porn, the idea of being wanted and preferred to girls by straight men is just extremely arousing.
For some reason, the idea of a straight man being attracted to a gay man is sort of hot-it just fuels one's confidence and esteem. Gay men are always stereotyped as liking men who are straight.