I'm sure you've heard of the 'gender card' or 'woman card' or 'race card', it's like that, except for LGBT.
Strange type of card. Wondered if you meant
invalidity card, except for still not matching with LGBTQI+* in any way.
Doesn't match the other cards ("my difference is my (gender|race), and therefore that is the card I'm going to play"), back-forming from the apparent intent, it would more likely be a "sexuality card", perhaps? Validity is... either everything (a short cut to encompass
any/all states of being differently valid to the supposed norm, yet still valid) or nothing obvious at all.
Thus my confusion. "Has Antifa used homosexuals as their validity card?" was a bit of a
garden path, and though I see how it
might mean "Have they presented the inclusion of gay persons in their ranks as evidence for their being representative of a normal cross-section of society, unlike the homogonously hyper-hetero haters (and other factors of commonality) that are their opposition?", if that
is what was being said, it's obviously a bit of an in-group phrasing. I therefore have to apologise for my position out here beyond the group-limits and being somewhat befuddled by it all.
Carry on!