I want to say no, but for some reason I just remembered something that happened 18 years ago with my ex and his 'friend' so I'm saying yes.
Haven't thought about this in years but I'm as annoyed now as I was then.
It was my boyfriend's birthday and he had a party. I turned up at his house an hour before the party started and was told to go through to his bedroom.
Where I found his brother sitting on the chair and my boyfriend laid on the bed with his arm around a girl I had never seen before, who turned out to be his friend from somewhere miles aways who was staying the night. He had known her longer than he had known me, by about a year, and apparently they had always been cuddly together.
Neither one of them made any effort to move and I was seething with teenage-girl jealousy.
She spent the entire night sitting on his knee and feeding him bits of food and talking about things that had happened before he met me. I felt very excluded. After the party I spent the night alone in his bed. He spent the night asleep next to her in a room full of other people, including his two younger brothers.
So I doubt they had sex, but I still felt cheated on. I bet he kissed the bitch too. Bastard.
Haven't thought about that in years but remembering it now I really did feel as bad as if he had cheated on me. Which he did later on.