Just my experience as a husband. One night at dinner I worked up the nerve to tell my wife I had fantasies of her sleeping with a well hung black man while I watch. At first she was shocked and was said "really, you want to watch me with a black man" and said it was a nice fantasy but you know I can't do that. Well we got home and I convinced her to watch a video with me but I did not tell her it was a amateur interracial cuckold video and I picked out one I had seen before with a very well endowed black man and this white wife who was your normal every day housewife. My wife got into something sexy and we got on the bed and I hit play, mind you my wife has never seen a well endowed man before much less a well endowed black man.
Well to make a long story sort of short, when my wife saw that it was a married white couple with a black man I noticed her breathing became shallow. And when the wife went over to the black guy and pulled down his trousers and out came this huge black penis my wife turned to me and mouthed the words "wow" and I said "impressive huh." My wife nodded yes and I told her I was in awe of his size and she looked at me and said "he's twice your size." Yeah it was exciting to hear my talk like that.
The condensed version of this is we had sex that night and I whispered in her ear "you'd love to have that black guy fucking you with his huge dick" and my wife had this crazy orgasm as soon as the words got out of my mouth.
My point is I am straight but acknowledged to my wife how impressive hung black men are and she never thought it was weird, she obviously knows interracial turns me on so why not talk about it. Your girlfriend is not dumb, she knows it turns you on so delve deeper into it and try to enjoy the experience with her.