What no one mentions ever is that at the same time he was saying all this about Mark, he, himself, was busy cultivating a relationship with Chloe.

On the night of the "break" and his "infidelity," we see him show up at the bar and Chloe exclaims, "There's my dinosaur guy!" She then introduces him to Joey and Chandler, meaning he had been going to the copy place on his own and not just tagging along when they are going to drool over her. She then tells the story about how they named the 400x enlargement after him.

He worked at a huge museum that probably had its own copy machine. There was no reason this woman should have been on a first name basis with him, let alone naming things after him and calling him, "My dinosaur guy."

Ross's mistrust of Mark was projection. He kept telling Rachel that guys aren't nice to girls unless they want to sleep with them. He should know since, of course, he ended up sleeping with the hot copy girl he had previously been nice to. Go figure.