Is it normal to have sex with my girl friend's friend and keep it a secret?

My girl friend and Ive been dating each other for 3 years. We are both in our late 20s, co workers, and fell in love with each other at our first day at work (our company is pretty new, there were many vacant posts and we applied around the same time, I in the accounting and finance department and she in the marketing and sales department). It started with simple interactions, to dating, to us become inseparable lovers and she moved in with me last year, and we have had sex together almost every night since, no matter how tiring our working day was. For 3 years I was very committed to her and never cared about another girl. But 2 weeks ago one of her old college friend came to visit, and I was enchanted by her beauty (my gf is gorgeous, but her friend is beyond my beautiful explanation). Her dressing was also very provocative, she was wearing a tank top and short. She was extremely fair skinned, with long silky black hair, gorgeous blue eyes, beautiful large breasts and lovely slim legs, words are not enough to describe her beauty, she was a gym trainer and had an athletic built and everything about her made my dick hard. I couldn't take my eyes of her, and she seemed to notice but didnt mind, and my girl friend didnt pay much attention to that, as she was too busy chatting with her. My girl friend's friend was supposed to be staying for a week and I happily agreed.

The night my gf and I slept in our usual room, while her friend in a spare room. Next day i pretended to be sick, so my gf could go to work alone, and her friend promised to take care of me. My gf is a very easy going and trusting person so she suspected nothing. after she left I flirted with her friend and she flirted back. But she wasnt gonna give away easily, so I gave her the bracelet i would have given a surprise gift to my gf on her birth day on May and also all the cash I kept on the house (my share only, not my gf's). My gf's friend than jumped on me and started to kiss on me, and I kissed her back, it was so hot, we soon toppled to the floor and went at it for the rest of the day, we showered together and went at it again in the bathroom too. By the time my gf returned from work we cleared up all the mess. My gf was happy to see I was well (I wasnt sick to begin with), but became sad when her friend said she had an emergency and was leaving the next day. That night I sneaked into my gf's friend's room and had another go with her, we also promised to keep it a secret till death and returned to my room in the early morning while my gf was still asleep. It was a saturday, no work that day, so my gf suspected nothing when her friend and I woke up late on the mid day. Before leaving my gf's friend hugged my gf and also hugged me, with the last hug I once again felt her hot brest on me and sensing my boner, she patted my crotch (unnoticed by my gf), let go of me, smiled and left.

Now that was two weeks ago and neither me or my gf had a touch with her friend since, and I dont need to anyway also she lives very far away, we are in Florida and she is in Arizona, she only came in for a vacation trip (my gf moved to Florida with her family from there after graduating from college, and this was the first time her friend came to Florida, visiting my gf was only part of the plan, she likes to tour all over the Sates and has no plan to return to Florida anytime soon as she said she doesnt go to the same place more than once). My gf later showed pic of her old college friends including the one who visited, and non were nearly as attractive as her, also she had dated the most men in the college than any other girls there, and if anyone ever looked at her no one could think of another girl in their lives again, but she was cautious never steal lovers of her genuine friends her friends (as told my gf). Besides this friend of her, Ive never been deeply attracted to anyone other than my gf. I deeply love my gf and will continue to, and we are even considering marriage. While it was a hot time with her friend, it was just a one time deal and the only time I cheated on my gf and I will never do again. I am glad my gf never found out, and never needs to, I dont want her to feel betrayed, so it will be a secret for life and I really love my gf and vowed to never do such again and remain committed to her for life . But I really don't know what will happen if her friend ever visits again someday...