Is it normal that i kissed with a man while having a girlfriend

I have a wonderful girlfriend, who I love to be with. This halloween however my feelings got complicated. We decided we would go as a couple to a party, dressed alike. As we had spent a great holiday in Germany, she suggested we both could wear octoberfest dresses. She made sure we both looked good and for me she aggegerated a little bit, with falsies, nylons etc. to make it look over the top.
At the party we were having fun with a group, until some guys started talking against my girlfriend and I stepped back a little. I leaned back against the wall making sure they wouldn’t do anything.
Then some guy dressed as a farmer moved next to me and started making compliments. We made jokes about what a great couple we would make and put an arm around me. So we stood for a few minutes, until he turned my cheek and kissed me on the lips. I couldn’t believe this was happening, but strange enough i felt comfortable enough to go with the flow. I could hear some people cheer at us, making it feel like a public event. Then he asked me if I would come with him to his appartment. I thanked him for the attention and went back to my gf. Of course she saw me and sees the irony of it. She tells me she preferres to see me with a man, than to flirt with another girl.
Now, a few weeks later, this moment keeps coming back to me, and I fantasize all the time about what would have happened if I had gone with him. I have to admit to myself now, that his kiss had somehow made me feel like his girl. My question: is it normal hat I have these feelings while I have a straight relationship..