"You can’t spell infinity without Finn!"
As pretty much everyone has pointed out, uh, yes you really can. That’s the point Rachel darling, you can’t spell infinity with Finn.
What I thought though, was that:
But not Finn as the name. Fin, like the french word for ‘the end.’
Because marrying Finn really would be ‘the end’ for her. Rachel would have ‘infinity’ but she would also be ending. Her life, her dreams, would be over. So yes, you can’t spell infinity without fin.