will i believe it goes randomly that's happened to me with my v4.5 even though they have made it more random on the v4.5's i guess you'll just have to wait and see!
well its supposed to be completely random. but i have heard that when the matchmaker comes, if you press a you'll get a girl. if you press b you'll get a boy... or maybe its the other way around?
But when you reset you tamagotchi, (make sure it has died) press reset and it will come with the opposite sex, the one you had before like if you had a girl youll get a boy.