I think the step stuff is popular because it's easy to resolve the dynamics of the relationships in the video from the least amount of words in the title. Its easy to tell who is assuming the social risk, who the aggressor is, etc... Then you can just use your imagination to change the relationships to match you fantasies. I think the most enticing content is the ones where the female assumes the social risk for nothing but sexual gain/pleasure. Not just using sex as leverage and/or to manipulate.