What to do when a fantasy becomes a nightmare [MFM]

I’m a blk male and I caught my white girlfriend in bed with a white male who’s penis was huge like really big , I never seen a dick that big in person and to be honest I never heard my girlfriend scream in ecstasy like she did and it was kind of a turn to watch. So I watched them for almost 20 minutes before saying something to let them know I was there and when they finally seen me I told them to not stop that this was better than porn lol. Ofc she didn’t want to continue and apologized again and again saying how sorry she was. Me, knowing I don’t have a big dick (kind of below average tbh) and not what u could call a BBC more like an abc or sbc was amazed that a huge cock could make a woman (my gf) scream and moan the way she did. I told her the truth, that I was devastated at first but after finding out that how much joy it brought her I made an agreement with her, that I would let her continue to fuck him and his huge dick as long as I could be in the room and watch. She was against it at first I assume she thought I was kidding around but once I let her know how serious I was and admitted how turned on it made me she agreed. We have been doing this for 3 months now and I loved every minute of it watching his huge white cock stretch her out and hearing her scream out as he reached spots my sbc couldn’t, but she told me the other day that she think she might be in love with him. Now I’m hurting and wondering if this is my fault for allowing it or is it just my penis size. I can remember her telling me the first time we had sex that she thought black guys had big dicks I laughed it off and said nope just a myth lol not thinking that it would turn it to this. My question is does size really matter? And could you love a man who doesn’t sexually satisfy you but would let you have sex with someone who does as long as he could be present? And where did the BBC myth come from? Because after seeing that huge white dick I know for a fact all black guys don’t have a bigger dick than white guys.