I feel like a rotten scoundrel confessing this (much less suggesting it), but I've discovered that female Miis get a lot more matchups than their gender counterparts.
For example, on Wii Tennis I've discovered that my chances at finding a match is 10%, meaning for every 10 attempts, I'll be lucky to find 1 person. I'm a star-10 player, so I understand that some lower level players are omitted from the match-making process. BUT when I use my girlfriend's Mii, who also happens to be a star-10 player, I'm guaranteed a match once out of every 2 attempts -- and not only that, but I'm more frequently matched up with a female opponent. On my Mii character, I haven't been matched up against a female opponent since I first started playing the game.
I don't know what the hell is going on or what kind of match-up system Nintendo's put in place, but it sucks. It sucks I have to play under my girlfriend's Mii to find opponents.
