I CAN'T SMELL RAIN
He wants to believe it's not true.
After all, Rachel didn't say she was certain, right? Rachel thrives on drama, and it would be like her to make a mistake like this – she hardly likes Quinn – and tell him. He knows Rachel wants him (and he kind of wants her back, much to his shame), so she would tell him this, even if it's not true, just to split him and Quinn up. That would be kind of awful, but Rachel's not perfect and she's probably talked herself into thinking it's true. So Finn has nothing to worry about, right?
But somehow, those ideas aren't sticking, or at least not anywhere near as effectively as the image of Quinn and Puck together is sticking to the inside of his brain. He never really got the whole hot tub thing; it seemed kinda weird, but Quinn said it happened and he accepted it. Because Quinn was Quinn; she was his girl and she wouldn't lie to him, not about something as important as this, right?
Finn grasps the thin metal of his locker, and vaguely wonders if he could crush it in his hand. He doesn't really think so, but with the way anger and hurt is coursing through his body, who knows. Rachel is long gone, so he can't run after her and ask her if she's sure. Or, more likely, break down and beg her to tell him its not true.
He doesn't want to think Puck and Quinn would ever hurt him like that. Puck has been his best friend since they were what, nine? Bros before hoes, although he feels an odd stinging sensation at that phrase for some reason he can't remember.
Oh wait, now he does. He said 'bros before hoes' and Puck just went on yelling at him, telling him he had to help Quinn more, and they fought. Even then, Quinn and the pregnancy was more important to him. Wonder why?
Finn shakes his head. No, he can't think that. That's a good sign more than anything; Puck and Quinn, if they did make a mistake and... God, he can't even think the word... they wouldn't be cruel enough to blame the whole thing on him, deceive him, saddle him with the responsibility. They'd have to tell him eventually that she wasn't his; when he faked being wheelchair bound for work (and royally pissed off Artie when he found out), when he was near-collapsing with sheer exhaustion, when he allowed Quinn into his home. They'd have to tell him it wasn't his duty. Right?
His breath catches on that one word. Duty. What the hell is he thinking? That's not a duty, that's his daughter, his Drizzle (and she'll never be called that, but that's what she is to him), and even if she was an accident, he loves her. He would do anything for her, so she's got to be his, because that would be just plain unfair – she's the baby he loves without ever meeting (and if Quinn goes through with her plans her never will even if he is the father), and he's not going to be tricked out of being the one to love her.
He paces toward Glee rehearsal with his fists clenched and his heart jammed somewhere up around his throat. He's mind is issuing frenzied commands: 'Find Quinn and Puck. Get them to tell truth. Make them pay.' Some part of him is saying this is a bad idea; that it would be easier to find out the truth subtly, spying on them, and if it's really not true he'll look like the biggest douche ever to walk the earth. However, he can't exactly listen to that part of himself; not with 'Can't believe you were man enough to knock up Quinn Fabray. Are you sure a real man didn't sneak in there and do it for you?' bouncing around his head like a hyperactive toddler.
As he approaches the room and Puck's mohawk comes into view, Finn has to resist the urge to throw himself onto the ground; scream and cry and throw a temper tantrum like he's three. He needs to confront them right now, and he can't break. Finn's not a smart guy, so there's a lot of stuff he doesn't know, and most of the time he's okay with that, but right now the pain and uncertainty is killing him.
He's built up this fantasy, where everything he's done and suffered for the pregnancy over these months has been totally just and worth it. Somewhere, he knows the it's true, and that the fantasy is about to disappear like the dream it is. But he needs one more second to pretend it's not. One more second, so he can offer himself to his best friend, girlfriend, and the fetus inside her womb, and let them break him in two properly.