Because reaching that level of hatred for something so innocuous requires at least a certain level of self loathing. Sure I can understand disliking gays, but to get out in the streets and make it one's life goal to hurt them requires a deeper level of intimacy with the subject, no pun intended. That's my guess.

I do disagree with the article on one point, the linkage of these activities to the right wing in general. I believe this is based more specifically on evangelicals. Many right wing people do not hate gay people. They may have principled disagreements with the legal specifics of something like gay marriage, but the real hatred comes from the religious (I do realize there is significant overlap, though).