Yes it ruins friendships
My best friend Erica and I have been best friends since grade school, We do everything together, we graduated together, went to college together, Lived together for a while after college and we have never had a serious fight, we have never gone more than a couple days without talking or hanging out until recently.
About three weeks ago we went to a concert together, Her and her boyfriend and me, He has never once been annoyed by me being around all the time, We all know each other well and as normal we booked a double room, We went to the concert and got pretty wasted, we ended up partying with some people and got messed up on a couple different things, we got back to the hotel around 4:00 am and Erica and Andrew were all over each other, They got into their bed and me into mine, They weren't trying to hide anything as they got undressed and crawled into bed, they were kissing and Andrew kept looking at me, Erica was the one who rolled over, looked at me and said "Come here", I quickly got undressed and crawled in with them.
It was wild and crazy and exciting, me and Erica kissed which we have done before but never naked and never more than that, we did everything, She went down on me, I went down on her, Andrew nailed her, He nailed me, Me and Erica were together for a bit while he watched until he got hard again.
In the end we were all covered in sweat and passed out with Andrew in the middle, I woke up around 10:30 am when Andrew crawled over me and rolled me onto my stomach, A couple minutes in Erica woke up and joined in, We spent another half hour going at it, We were all sweaty and gross and Erica suggested a shower, She jerked Andrew onto my b**** after he did me doggy.
We drove home and I stayed the night at their apartment and we banged all night, the next day I had to go to work and they were going to his parents place for the night, That morning was the last time I seen Erica face to face and I have only spoken to her on the phone a few times, my concern is that she has regrets about what happened and I don't want to lose her as a friend over something we can make better by just sitting down and talking.