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Remember JUSTIN GUARINI . . . the guy who lost to KELLY CLARKSON on the first season of "American Idol"?  Well, he's married with kids now . . . but he's claiming he was quite the playboy back in his "Idol" days.


At a recent performance of his one-man stage show, "Lovesick", Justin told the crowd that he and Kelly started hooking up while they were filming their HORRIBLE post-"Idol" movie, "From Justin to Kelly".


He didn't say how long they were together . . . but it was short-lived.  He basically said he was left behind when Kelly's career started taking off, and his didn't.


But Kelly wasn't Justin's only conquest.  Supposedly, he also had a "fling" with TAMYRA GRAY, who came in fourth that season.


Kelly hasn't responded . . . so it's unclear if she remembers who Justin is.