I'd suggest them directly, but I keep having problems doing so.
Honestly, I really don't see the need for this particular tag. Pretty much all relationships/marriage/booty-seeking quests are counted as a side-quest in relation to the main story in games that have these options. We already have a tag for that, and it is a synonym for quest.
I doubt anyone is going to be searching for only relationships related questions, and anyone searching for such questions regarding the sidequests within a subset of tags should be able to find it under quest.
Our tagging system has changed drastically since this post's inception; it no longer makes sense to have a broad relationships tag on questions.
Instead, if a game's relationship system is a big enough topic area that it warrants it's own tag, consider creating a game-name-relationships sub-tag (substituting 'relationships' for whatever the game calls it).