Kate,

It seems pretty simple to me.

Most straight porn is bought by straight guys.
Straight guys don't want to see dudes fucking each other in their porn, but they like g/g or b/g.

Therefore directors, conscious of the fact they have to sell product to survive, cater to those needs and leave the bi stuff out of it.

Their mission is to make money selling fuck flicks, not provide social education on the virtues of tolerance and man-love to the great unwashed masses. Who cares if it's discriminatory, it's what the market wants.

Yeah, I know there is an argument that more girls might buy it if it's bi, but there is enough bi / gay stuff to cater to them already.

CM