Honeymoon Chaser

A person who only dates an individual until the "Honeymoon Phase" ends, in which they seek out another partner whether or not their current relationship is still ongoing.

Erick: Hey Sam, you still dating Serena?

Sam: Nah man, found out Serena's a honeymoon chaser. I think she's dating Jerome now.

Erick: Damn, that's almost as bad a thot.