Okay first of all, before I say anything...
*I do NOT want to sound sexist.
Now, I do think that more girls may like Tamagotchi's more than boys because the characters are more "cute". And I'm sure Bandai has noticed that since they made a design called "Girls Rule" or "Girls Rock". Something along the lines of that. And in the commercials, they are a bit portrayed as a toy for girls, because there are at least one girl in the commercial.
I remember one boy that once went to 3rd Grade with me. I had a Tamagotchi, and I brought it to school and showed him. He thought it was cool, and went out to buy one. His friends spotted him playing Tamagotchi with me, and started teasing him, saying that he was playing with a "girl toy" and that he was "turning into a girley-girl".
I personally think that Tamagotchi is a uni-sex toy. Anyone can play with it, and they should be able to without people making fun of them ^_^