My gf and I are both 16 y/o and have been together for 4 years. I work part time with my cousins at my uncle's garage after school. Usually our girlfriends and friends come to the shop after school with us and hangout. I was in the storage closet at the back of the garage today, looking for a new alternator, when my cousin's gf came up behind me, put her arms around my waist and started kissing me on my neck! At first I thought it was my gf (cause she does this a lot), so I turned around and started kissing her but then I realized she wasn't and I pushed her off. I asked her what she was doing and she just smiled, told me she "loved" me, and then left? I'm like wtf? Then tonight she texts me and asks me if I wanna "hook up," and I said no. Then she asks me why not and I say cause I love my gf and I'm not going to cheat on her, and also because she is my cousin's gf. They have been together for almost a year everybody knows that she has a crush on me even though she's dating my cousin. He said he was fine with it as long as she didn't "cross the line" (lol?). Correct me if I'm wrong but I'm pretty sure this "crossed the line." I'm thinking of telling my gf and my cousin but before I do, did I technically cheat on my gf cause I kissed her back (thinking that it was my gf who seduced me)? What should I do?