I'm going to be generalizing a lot here, though I am aware there are many exceptions, I'm just illustrating my point.
Straight men love sex involving pretty women and THEIR PENIS ONLY. They don't want any other penis involved in the situation, that's what makes them straight! Thinking about lesbian sex, that works out well for them. Gay sex? That breaks the above rule.
So if a girl is bisexual she can be more easily pictured having sex with another girl, that makes her more attractive.
If a man can be pictured having sex with another man, that is where the 'gross' part comes in. Straight women don't usually enjoy gay porn as much as straight men do lesbian porn, so what makes a bi girl 'hot' makes a bi man 'not' in the eyes of many. Straight women tend to favor strictly hetero situations.
Personally I love gay porn, but I prefer lesbian. Actually, I enjoy the thought of 2 men together as much as I do 2 women, but because of the high demand for lesbian porn compared to the demand for gay porn, very often the production quality is lower for the gay porn. It can simply be there is a surplus of beautiful female porn starts that there is enough for all genres, while with men the most attractive are so often in hetero porn.
As usual, money makes the world go around. Great lesbian porn will bring in more views than equally good gay porn, because of a larger consumer base. Therefore they get the better actors and production apparatus.