Maybe they were both at Xavier's around the same time? Like Mark's senior year would've been Julio's sophomore or something. He couldve been like, "this dude, again. :|" as soon he sees Mark again in X-Factor. I'm having it set that Mark's only been with X-Factor for two years. He was a slave to the club scene before he finally decided to team up with a team so in a way he's still getting a grip on this whole hero business.
As far as Julio accepting himself, Mark could definitely have been a supportive friend. Or you know, just been there all, "COME OUT ALREADY. WE HAVE COOKIES." Unless you had something else in mind?