I was drunk and I cheated on my BF

Yesterday I went to a house party. My bf couldn't go with me cause he had some family reunion to attend. The party was filled with people I knew so I felt quite safe. A couple guys tried to make a move on me but I just walked away and it was fine. I wait in line for the bathroom and I see my best friends boyfriend leave the bathroom. I enter the bathroom and my best friend is there. I do my thing and I'm washing my hands when she starts flirting with me. So I asked "do you wanna makeout?" And she responded no cause she had a boyfriend. I just shrugged and she started applying lip gloss on me. I didn't get why because I was mostly brain dead by that point. But then she started kissing me. We made out for 20 seconds and then she just left. I had never kissed a girl before, now I am sure I'm gay, I feel immensely guilty that I did that to my bf, and I don't know what to think or do. I might just take this to my grave.