I mean, students have been dating and even marrying professors for forever. And why wouldn't you flirt with people at a political event when something so central to who you are (your personal politics) have both brought you to the same event?
From what I can tell, none of these relationships were with his actual students, just students at the school he taught at. I can't tell you how many successful relationships I know of in that category, especially in the grad student age range but occasionally in the undergrad level as well.
I don't know what the definition of "grooming" is in this situation, nor do I understand how it's an abuse of a power structure, if he's not dating his actual students. What's grooming, just fawning over a younger person in a relationship? Why do these young men feel unable to leave the relationship with him? He has.....no power over them, except the normal power of people in a relationship who don't want to get emotionally hurt by breaking up.
I think the student group acted a little eye-rollingly, but I also acknowledge Greenwald and Taibbi have become weird "we must police the Left" type idiots who claim to be liberal but only repeat alt-right talking points. I don't think there's any "latent progressive homophobia" here.