Should i cut off all ties with this guy too keep from hurting others?

So on New Year's day me and my best friend hooked up... We are both guys and we are both straight. I have always had ideas about being with a man but never followed through with anything until now. The guy i was with is my best friend let's just call him G. So G and i partied the night before with all of our friends and all of us went back to my house because my parents are in St. Thomas. So after all of my friends left G stayed back to clean up and hang out a little. G and i have been friends for a number of years now and i have always thought of him in a little bit of a different way. He is extremely attractive, a burly football player and somewhat of a womanizer. He has always been "hands on" throughout our friendship and is very touchy feely and is always hugging or touching me. I just thought that was how he was because he is a football player and kind of a frat house type. (I play tennis and have never played football so i don't know how football players interact) Anyway, when everybody left G and i were cleaning my guest bedroom and G cornered me on the foot of the bed and said "I didn't get a New Years kiss last night." and then he started touching me, i was hesitant at first but let go. I will let you fill in the dots. If he and i just had a fling and all we were is friends then there wouldn't be any problem. What the big problem is he is my best friend's boyfriend. This girl that G is dating has been my best friend for almost 10 years. I feel bad for what i did, and now G is texting me saying that he wants to try it again, or just hang out "alone." Not only do i feel guilty with my friend, but it is also really weird because G is such an alpha male type and he would never strike me as doing this. Even though i have my hesitations he is still G, and he is super attractive, built, and he is very smooth. I just do not know what i should do. Do i continue with this fling? Or cut off all behavior?