I personally think that he's straight because of the girls he's liked but because he's so young he's probably sexually confused. He admitted to loving brian but only in a best friend kind of way
This is probably the most accurate answer your gonna get…
Stewie is bisexual, so he likes both boys and girls but he doesn't like girls as much as he likes boys. Technically that makes Stewie 80% gay and 20% straight if that makes sense. :)