The thing is though there are pretty good examples (not of mages or anything) but of specific groups of people hurting an individual and them forming a permanent bias against that group. I mentioned this a long time ago but there was a solider who had been tortured by a group of Al Qaeda. In his psych profile before he left the war he had absolutely no bias against -well any group really. When he came back though he loathed all middle easterners irregardless of religion or the fact that none of them were members of the organization. He wasn't willing or able to differentiate between the people who had hurt him vs. People who looked similar or came from that area. He's still struggling with this and has a deep seeded fear, resentment and hatred to everyone he views similarly. Attempts like having him meet really kind individuals who were smart and sympathetic to him from that area had absolutely zero impact on how he felt towards the group (even though this man was well above average intelligence and a good natured, kind hearted, open minded individual). Now your probably thinking "Kirrahe that was one guy!" This study had over 500 similar participants and almost all of them had similar responses to this one man. If I recall there were 4 people who could get over the feelings of fear but in a group of 500? That number was so low it really surprised me. I'll see if I can't dig this study up (I was in college so it was a while ago lol) it's pretty interesting, and there was another one about Vietnam soldiers I believe. One of the reasons you see the news story's about forgiveness is because it's so rare.
What I'm trying to say is this, yes Cullen could potentially be able to distance himself from his fears and anger to the point where he could be very good friends with or romantically involved with a Mage. But it would be very very unlikely in the real world. Without professional counseling it'd be that much harder. The brain is designed to protect itself from further harm and damage. Becoming fearful and resentful is the best method the brain has to keep you far away from what it perceives as danger even if it is racists or biased. Also people naturally find love and biologically want to procreate. A person may have serious reservations about having children with a type of person they fear and it's probably double the impact because his children would then also run the risk of possession. (This is in regards to a straight romance obviously)
The truth is, I think Cullen will be able to romance a Mage in the game and that they will have many sweet lines about "I hated them till there was you!" And other such nonsense. But this is a game and that isn't realistic. Yes I get it, it's a romantic idea about love conquering his fears and making him a better man -but it isn't likely that could ever actually happen. Now if anyone is looking forward to a Mage romance that's great! I'm just a curmudgeon who doesn't want people to have silly idealistic views on how trauma actually behaves and manifests because there is already so much misinformation out there. I should also state that the permanent bias against other is ABSOLUTELY NOT TRUE FOR ALL PEOPLE. So if thinking Cullen is one of those rare cases helps when romancing him, go for it!