I think it's worse to go on the date! You aren't interested in them, and you are going out with them? For how long do you pretend to be gay/straight (whichever you aren't)? Do you stop when you're about to have sex? Do you go full into a relationship because it's rude to finally turn down someone because you have a sexual orientation that they aren't part of?
No, you are leading them on and not interested in them, and it's rude to say yes. You can let someone down nicely, but no it is not shallow nor sexist nor anything to turn someone down. You don't even need a reason, sometimes it's just not the right time in life and you don't want to!
Absolutely not. You can't help what you're attracted to, whether it be the opposite sex, same sex, or both sexes. You're not shallow for having preferences like every other human being and knowing what you want. I can't imagine anyone seriously voting "yes" on this, unless I'm somehow mistaken on what this question implies.
Using the word shallow to describe physical attraction I don't think accomplishes much because using a definition like "not caring about or involving serious or important things" is completely subjective so it can just vary from person to person. I don't see the point in a debate really. I think using the word shallow is a word to try and cause a person to feel scared by feeling socially weird. The word preference can be used instead of shallow and I have no issue with a person having their personal preference or being shallow relative to whatever they think it is.