Um being bi means by definition one isn't gay or straight, that's the whole point... if someone 'comes out' as bi they are by definition 'declaring' they aren't gay or straight. So not sure how that doesn't work.
Right, but you could live your whole life as bi without stating anything and it wouldn't change the fact you are bisexual. 'Coming out' doesn't mean 'being'.
Whereas your gender is what you declare it is, there is no further definition than that. No test, no way of telling what gender anyone is other than the label they give themself.