First off, the 'Kinsey Scale' looks like a bored statistitian's lunch break argument for why his straight buddies should let him give them a blowjob; no mention of the 'did it / wanted to' issues, or availability of male and female offers. Ignore it.
Second: being a male porn performer is bloody hard work, and not a lot of men are cut out for the pressure of maintaining a hard-on in front of the camera and crew and constant interruptions. The ones that can perform, and reliably so, are mostly the ones who can ignore the distactions and focus on their work; meaning, doing a guy up the butt instead of a chick is merely just one more distraction to ignore and focus on whatever internal imagery gets him through the performance.
Keep in mind, I'm talking about feature porn movies, here; not the labels that spit out so many new episodes in a series every month.
The observation of more guys in straight porn going for the ass; I had a discussion about this with a straight buddy of mine, recently, and his take on it was that it comes down to 'getting noticed' in the flood of new porn productions entering the market. The trend is that things grow increasinly extreme, or fringe, or otherwise show something more than what has been shown before. Going for the ass is just one such 'something more'. It doesn't mean that straight guys in real life want to do it in the ass more than they used to; only that the ones who do want to can express it more freely, since it's become so visible in porn.
That said, I too watch straight porn from time to time; not sure why, exactly, because I always end up thinking it's bo-o-oring. If there's actually a proper story-plot, the action tends to be just lame; and if there's quality male beef, the chicks are often either limp-breasted and worn, or worse, overdoing the 'bedroom eyes' gig to the extent that they look half asleep or just plain drugged. And more often than not, the camera goes to great length to not show male anatomy any more than absolutely necessary.
And then, along comes the one that gets it all right so it just works for me, putting all my complaints to shame.
But all in all, it appears to me like gay porn is higher quality on average.
Or, does it just so happen that I tend to find the wrong titles in straight porn?
~ Neo