I think Suigetsu and Karin having a relationship with each other isn't so unlikely. Obviously Karin hates Sasuke now. On the other hand, when Suigetsu and Karin were fighting, they certainly were interacting with each other more than Sasuke with Karin. Also their fighting may have been actually tension and denial that they liked each other.I'm not saying it's likely, but I think it's possible.