He doesn't quite remember the moment where it progressed from "secretly crushing on Gerard" to "hey it's not so secret anymore". Mostly because that moment is usually delineated with a kiss, and, well, Gerard plays it up onstage. She always has, and probably always will, and basically this means that between the two moments, he'd probably kissed her a couple hundred times.
She kisses nicely, too, all hot, wet, and open-mouthed, and thank God for guitars (excellent boner shields. Slamming the guitar into said boner while flailing around isn't quite so excellent, but hey, at least he knows first-hand that dicks can and do bruise). And once she files you under "okay to casually kiss", well, a lot of her other physical boundaries come down, too.
Gerard Way snuggles like it's going out of a style, and Frank thinks it might be the best-kept secret in all of Jersey. (Considering the gossip network, it may be the only secret in Jersey.)
But the point is, every kiss is good and he's not sure when it stopped being just stage-play and turned into casual-friends stuff, or when it stopped being casual-friends stuff and started meaning more. All he knows is that sometime between touring on Bullets and making the video for "I'm Not Okay (Trust Me)", the dynamic shifted and shifted and it seemed like between one moment and the next, he gained a hot-ass girlfriend.
He's not complaining, though. (Actually he thinks he's fucking lucky.)