Why are you in an OOC romantic relationship with your character's "mate" anyway? Mates are a roleplaying thing- your character, in roleplay, is mates with that other character. It shouldn't be an OOC thing at all- you're not in a real relationship, so why is it an issue? If people want to get mates for more than one character that's their prerogative, because what if they want to be in more than one roleplay, and in that roleplay, their character gets with another character? It shouldn't be an issue- the player themselves is not bound by loyalty to you. FeralHeart is not an OOC-based dating site, it's a roleplaying/chat game.
Also, you should not have to know a person OOCly for a while before your character becomes mates with their character ICly- because it's not even an OOC relationship, it's a roleplay.