... How can you both understand and not understand at the same time? Are you american?
To sum it up, minorities suffer from the gaze that is cast upon them, especially if this gaze is exploitative, degrading, objectifying and made by and for the pleasure of the oppressor, thus reinforcing the oppressor's bias against the minority.
If trans porn is the only way these men are confronted to the existence of trans people, they'd oversexualize the idea of trans, which in turn leads them to blame/despise the trans even more (because of both their conservative views and unhealthy relationship with sex), not even leaving trans people the space to be something else in their eyes than objects to jerk off to (aka "people").