Ok, this might sound weird, but from my perspective - In these types of games, you are able to play out someone that you can't be irl. All of my characters are facets of the same person - moi. Since my characters all have some nugget of me in them (to varying degrees), I have no issues with having them all romance Cullen (except the two that went for Bull and Blackwall). I've never felt the "OMG that's so Ravenna's man" when Raven romances him, or any of my girls. Cuz they're all me! 
P.S. I'm the same way with my various Sheps for Garrus, and my DA:O girls with Alistair.
Yeah, I get that perspective from a theoretical point. I just don't do self-inserts, myself- I roleplay different characters- so I never feel like they're me. All my heroes are just characters, completely fictional people with their own very different lives and quirks and beliefs and morals, and almost none of it is taken from myself. I like to explore the aspect of "someone that you can't be irl," as you mentioned, only to the other spectrum - I can't be a fiercely religious believer, or a shady criminal manipulator, or a rich arrogant noble etc myself, but I can roleplay a character like that. I create them as one would write an original character in a novel, shaping their actions and choices like a story... and that sometimes ends up in the love interest being locked into that one story because it's the one they belong to in my head. Say, Cullen, he doesn't belong to me - he belongs to this specific narrative.
I can romance certain love interests with more than character - I've romanced Zevran, Fenris, Sera, Kaidan, Thane, with a number of different characters, and never felt that they "belonged" to one more than another. But then every now and again I get trapped in this mental space that this love interest belongs to that one character and that one character alone, and I can't move myself out of it. I couldn't tell you why, or what causes that, it just happens. Unfortunately Cullen seems to have fallen on the second spectrum for me; I can see him with Ari only, and all my attempts to romance him with anyone else have either fallen flat or died out after the first flirt. Or is it fortunately? I can't tell - but I do feel that my "locked" romances are somewhat more meaningful to me. I could wax poetic for years about Evelyn and Alistair, Rowan and Sebastian, Ariana and Cullen, but if you asked me about my favorite Fenris romance I would blank out on an answer. There were many, I loved them all, but don't really have a favorite.
In any case, I'm still trying. Who knows, maybe one of these days I'll manage to find the sweet spot for his romance with someone else too. If not... well, there's always that strategic save right before his door to give him a nice woo-break with Ari any time I start missing his romance too much. 