White male here using a throwaway for obvious reasons, just wanted to say that I buy OP's story as someone who went to an SEC school himself and was in a fraternity there. You will notice that a lot of the top houses there don't really have any minorities at all, these are the same houses that throw the biggest parties, have social events with the hottest sororities, and have a sizable role in leadership positions on campus. Many of these houses had brothers who were pissed off whenever a black football player would hook up with a rather attractive looking white girl but, for obvious reasons, these guys kept their attraction held in check but the racism came out behind closed doors.

Most of these guys go into finance and are used to being on top, they had rich parents, grew up in the best neighborhoods, and had it all in their college years. The second they see OP with a beautiful blonde by his arm (Fraternity brothers are obsessed with those kinds of women btw), they can't comprehend it. In their eyes, OP is a minority and someone far below them. In college we were the ones sleeping with hot girls while that nerd was studying for his physics exam, how in the world did that happen?

The guys OP is encountering see themselves as above him and they hate the fact that their trophy (his girlfriend) is by his arms. OP, you would be best to move because it is not a good idea to fight a battle with these guys. Many of them are well connected and can get away with a lot of things due to their social backgrounds.

As I have moved out west and made friends with various races, I have become an accepting guy. I also know of men of various races that date white girls without receiving issues for it.