While I am deeply sympathetic to any women getting harassed by other members, I am completely against this idea, even an ostensibly voluntary form. There's no benefit at all to verifying one's gender or sex as 'authentic' as all genders and sexes are equally welcome. Transgender and intesex people will be further marginalized and subjected to scrutiny which nobody on the planet has the right to impose on them. To say nothing of the privacy invasion this would create as users, all of whom I expect would be women suspected to be men, would feel pressure to step outside of their comfort zone in order to gain acceptance. To say nothing still about whether the site operators are themselves trustworthy with that kind of information (sorry, but running a cool site counts for exactly nothing in my book).
Trust and reputation are earned over time, both in real life and on forums concerning delicate topics, and there is no way to speedrun this process without creating an especially toxic and pointless version of Checkmark Twitter, and risking the site's open and relatively welcoming environment by stratifying users into "somebodies" or "nobodies" based entirely on their risk tolerance, attractiveness, access to animals, willingness to produce content, or whether they're famous or well connected.
If someone can't fap to something you post without first being sure you meet their standard of beauty or orientation, that is 100% their problem, not yours. If someone wants to meet you but doesn't trust you are who you say you are, you are not obligated to do anything special, and they can fuck right off out a window.
My name is àqua, I am a blue sphere with Opinions, and my gender is, apparently, a unicorn emoji. I am not concerned with credibility, and I think you shouldn't be either.
?