Out gay people are both more likely to out someone and more likely to have complicated feelings about others preferring not to be outed.
> What counts as absolutely necessary?
It's always better to come out than be outed.
> How much of a role does flirting play in the business dealings of founders?
Ideally none. Unfortunately when you're a woman in her 20's courting men in their 30's as investors, the rituals often overlap with romantic overture. (suggesting coffee, discussing deals/valuation over dinner, feigning interest in their opinion or soliciting advice, grooming, etc.)
The point is not that it's ok to flirt, the point is that it's ok (and not your fault) to be perceived as flirting. Too many female entrepreneurs avoid approaching male VCs because they're afraid of being misinterpreted.
Your broad point is a good one, and I'll note it is not completely asymmetric by gender either. Anyone with functional communication skills should be able to resolve this potential for misinterpretation fairly early and painlessly.