Men need to see a penis in straight porn (presumably to stand in for their own), but not one that is attached to a guy who might be threateningly attractive, not to mention plausibly appealing to the woman involved. Maybe this insistence on a male blank slate (a kind of reverse objectification, when you think about it) makes it easier to project oneself onto the disembodied penis, but it also protects men from the potentially scary experience of being turned on by both partners of a heterosexual encounter—which, yes, does involve another dude. In other words, the bland interchangeability of the “unreasonable” looking men allows them to avoid confronting the terrifying specter of homosexuality.