He dates Quinn while having a bit of a thing with Colton — he uses the same argument that Santana and Brittany do of “it’s not cheating if you have the same parts”, but Colton refuses to help Finn cheat.  The tension is still there but Colton refuses to ever let it cross a line because Quinn is one of his best friends and also he’s just very very very anti cheating and isn’t going to be a part of that

After Quinn and Finn break up, they start actually hooking up (though it does matter to know that it’s always Colton touching Finn, Finn still has too much internalized homophobia to ever touch another guy’s dick), Finn is very hot and cold about it but Colton is fucking in love and at this point he actually believes that this is a relationship and that Finn just isn’t ready to be public about it, but Finn isn’t nearly as serious about it.  And, when he starts to feel like it’s becoming serious, he freaks out and sleeps with Santana to prove to himself that he’s still straight (power of madonna).  

also side note Finn sleeps with Santana within like 48 hours max of Colton singing a song in front of the entire club that basically boiled down to “I’m fucking in love with you and I think I’m ready if you are”

He doesn’t tell Colton about this, pretty much the entire rest of the club knows but no one tells Colton because they all assume that Finn would have told him first.  But Colton has no idea, things continue on, and Colton is generally pretty content even if he doesn’t like feeling like a dirty little secret.  This continues on right up to Theatricality, where Finn and Colton have sex for the first time.  Finn completely freaks out afterwards, lashing out at Colton and breaking his heart, and that night is when the lamp incident with Kurt happens (Colton finds out about that and it’s one of the few times that he completely loses his shit)

Fast forward into season 2, Finn is dating Rachel, Colton is trying to be supportive because Finn is still his best friend and he and Rachel are actually really close too, but it’s also breaking his heart

Fast forward to Furt... the wedding is a wild time lmao.  Before the ceremony, they’re all waiting around for the song, and Sam asks Colton out.  Colton half rejects him ("you're incredible, Sam, and I really like you.  But... there's this guy, I'm still kind of hung up on him, and it wouldn't be fair to use you as a rebound.  I won't ask you to wait for me, but if you happen to still be single when I can be sure that I'm saying yes for you and not to spite him, then I'd love to"), and Sam is very chill about it 

Towards the end of the episode, Santana tells Rachel about sleeping with Finn, Rachel makes out with Puck and then tells Finn, who breaks up with her (all still at the wedding), and now Colton is literally the only person in the club who doesn't know about the Finn-Santana hookup.  Finn tells Colton about the breakup but not the reason, and they hook up after the wedding

Colton thinks that Finn is finally getting over his shit, but Finn is definitely using Colton to get over his heartbreak and to feel better about himself (his girlfriends might have both chosen Puck, but Colton chose him), and their friends with benefits situationship starts up again

We get to special education (which I’m giving a week or two before we get there), Colton and Finn have been hooking up, things are mostly okay, and then Schue announces his plan for sectionals.  Rachel and Santana still have their fight, but since Rachel already knows about the Santana-Finn situation, it’s a tiny bit different, and instead of Santana bringing it up, Finn tries to tell Rachel to calm down and Rachel yells at him for “only defending santana because they had sex”

So now Colton is like “I’m sorry, what the fuck”, then Santana yells at Rachel about how she cheated on Finn with Puck, and Colton is like “what the fuck” and suddenly a lot of things are making sense to him and he doesn’t like the image that it leaves him with.  Rachel storms out of the room, then everyone else quickly leaves because clearly no rehearsal is happening today, and Finn is standing there totally lost and confused; he wasn’t even dating Rachel at the time, why is it such a big deal?  Then he sees Colton still sitting in his chais, face totally blank, seeming like he hasn’t even noticed everyone leaving.

This leads to a huge Colton and Finn fight.  Colton has been biting his tongue while everyone was there because he’s not going to out Finn, but as soon as it’s just them, a fight breaks out ( @megdonnellys plotted like half of the fight for me and it’s incredible!!!! I don’t want to send this post off the rails but I’m def happy to share if anyone wants more details!), but it ends with Colton accusing Finn of cheating on him

The next day, they’re in the locker room before football and Finn basically tells Colton “I never cheated on you because we were never together” (credit to halli for this devastating conversation too!) and Colton is heartbroken.  But Colton has also spent the past two weeks thinking about Sam and realizing that he does like Sam and thinks that he could be very happy dating him regardless of the finn-of-it-all (and in fact was already thinking that he’d be happier with Sam than with Finn treating him like a dirty secret), and he goes out onto the field and tells Sam that he’d love to go out with him, and they kiss

Finn and Quinn still get together at the same time as canon but without the cheating, then they break up at prom and Finn spends the next few weeks thinking about Colton and Rachel and Quinn and sorting out all of his feelings for all of them and realizes that oh fuck, he’s actually in love with Colton isn’t he

And then he kisses Colton at Nationals, and they date from that point on!