Grindr is mostly used for semi-anonymous "hookups" - I am told anecdotally by people who used it and by the media at large - which is something even out and proud people might not wish the world to know (e.g. because their spouse might disprove), especially not in harsh detail. A lot of the communication on grindr ought to be pretty explicit too given how it gets used, something you might not want to see made public either, even if you're out of the closet already.
Now the other theoretical site might be as "bad" or even more so as grindr (the fetish website you suggested), or it might be a "tame" website used mostly in a "tame" manner, which of course still has "potential" to a blackmailer, but less so than the grindrs, tinders, backpages, adultfriendfrinders, etc of this world.
[1] https://en.wikipedia.org/wiki/Recruitment_of_spies#Love,_hon...
Simple explanation that likely explains the security panel's reasoning behind their statements.
Also explains why a phone number as an identity token is a horrible idea.
why would grindr have that? Hell, they dont even validate the random string you can put in the email field
select opm.name, opm.address
from opm_security_clearance_files opm
join grindr_users grinder
where grinder.phone_number = opm.phone_number
and opm.clearance in ('SECRET', 'TOP SECRET', 'SCI');
If any of those people are in the closet, they could be susceptible to blackmail.