I don't necessarily think that we should use heterosexuality as a "default" sexuality. It is simply not defined. I'm probably over-examining your reply but I think a more accurate way to express things would be to say, "then she is most likely straight" rather than saying definitively that she is 100% straight. What we DO for sure know is that she is attracted to men. Personally, I just don't consider a person's sexuality until it becomes a topic addressed by the writers.