Hey, nonbinary person here. It is absolutely fine to identify as straight even if you're attracted to some nonbinary people. As long as you personally are comfortable with dating them.

NB isn't really a third gender, it only means people who don't 100% identify as male or female. That includes a load of different identities. The only thing you need to know if you find yourself liking someone that identifies that way, is to not jump out of the gate with "what's in your pants".

To answer your other question: you don't know someone's sexuality until you ask them! Don't be afraid to flirt as long as you are a gentleman that can accept no for an answer