Men have an almost imperceptible rush of adrenaline when they see a potential mate with a potential competitor. This is a biological enhancement to make the man perform better in order to beat the competitor and lead to their own offspring.

The thing about "women with penises" is the man feels this same rush of adrenaline, because they see "women with penises" as a potential mate, and the penis is recognized (unconsciously) as a potential competitor, yet there is no threat of an actual competitor, because there is only one party: the "woman with a penis."

This, along with novelty, is a big part of why men, and often traditionally straight men, are attracted to "women with penises:" the biological preference for competition