I think you just have to try to go with what you think fits and is most prevalent in the group. A lot of pictures can fit in many categories but there usually is something that sticks out at the end.
If you had a Redheaded, Big Titted, Pierced and Shaved, BBW Asian Woman getting screwed up the ass by a by a Large Cocked Black Man who was also getting screwed up the ass by a Transsexual with a Huge Dildo up her ass, you would have a lot of options. But, you wouldn't choose a softcore category. Even if some of the content was the 3 of them getting undressed.
My point...

I think you have to go with the final outcome of the set.