On the V4 & V4.5, and the two Japanese models (EnTama & UraTama) that they are based on, most of the characters that you can raise are available only with a pre-defined gender.
Let's look at Mametchi for example. On V4, if you want to raise a Mametchi, the baby you start with must be a boy. However, on previous Tamagotchis, you could raise a boy or girl baby to be a Mametchi. On a V3 a Mametchi could marry a Mametchi if one was a boy and the other a girl. Since Bandai made it so that the V1, V2, V3, V4 and V4.5 can inter-marry, the V4 had to allow for a girl Mametchi. The list of characters that the V4 can display includes Mametchi. (Those that can't be displayed appear as Nazotchi.) For V3 compatiblity, Mametchi must appear in the boys list and girls list. The Matchmaker probably just picks a character randomly from the opposite-gender list. Thus, she could bring a female Mametchi.