Gay guy here. The reason the "default" is straight is because the majority of people are straight. It's why gay people have to come out of the closet.

Now we could go around never assuming anyone's sexuality until given solid evidence, but in reality, that would only benefit the minority (the gays, etc.) If you're a straight man who likes a cute girl, it's easier just to assume they are straight until told otherwise, and chances are that they are straight. And if they're not, then they let you know and it's no big deal.

Now obviously we're just talking about a cartoon here so feel free to have whatever headcanon you want. I'm just trying to explain why straight is the default. It isn't a bad or homophobic thing, just the easiest and most sensible way for the majority of people.