I can definitely see that happening. Mark is the type of person to enjoy a good party despite the occasion and when he was younger he was not shy about getting out of hand. Maybe Mark noticed how much more comfortable Julio got while they were drinking and one thing lead to another and bam they made out before Julio became uncomfortable and skirted off? Mark's sexuality was never much of an issue for him. He embraced it and just owned it when he came to realization that he's gay.