Is keeping a secret lying? Well, maybe that depends on the transaction the person is entering into. If straight women did not lie about all manner of things, big and small, when entering a relationship, the world would certainly be a different place than it is now. (Would using makeup be considered as lying?)
The question of concealing something to do with gender identity, would, if it is considered a lie, mean that presenting oneself as one gender, yet having some 'legal defect' or qualification concealed yet present in it, is a violation of the implied statement you make by presenting yourself as implicitly straight. Does this mean you have to present yourself so what you see is what you get?
The traditional cure for this is getting to know someone really well before you get involved much with them, but today, we are in such a hurry and think we know better anyway. Haste not only makes waste but offers a protection or shelter for the lies of others. That way we act as enablers, perhaps in some sort of co-dependency.