They may think their "no fraternization" rule prevents workplace romance.
Actually, it prevents union organizing.
They're not allowed to prevent union organizing.
This is a blatant violation of 29 USC 157. Read it.
You engaged in "other concerted activities", "for the purpose of" "mutual aid".
It doesn't matter whether you discussed labor organizing, all that is required is that you might've. Anyway, your meeting certainly was about employees looking out for each other, which is the very seed of union organizing. And that isn't any of their business anyway, you don't need to justify that by telling them you're labor organizing, in fact they're specifically not allowed to even ask, or otherwise snoop on your “whatever it is that might be labor organizing".
Really. This conversation isn't allowed to even happen.
"you were fraternizing with Kalpana outside of work. Reprimand! Read the handbook."
"Kalpana and I were labor organizing".
"Oh, that's all right then, reprimand canceled, by all means carry on."
The above is not what 29 USC 157 means. It means they can't even stick their schnozz into what you and Kalpana were getting up to.
Unless it's something obvious that is reasonable to prohibit, such as a workplace romance. But they are not allowed to go on a broad fishing expedition for that, because they are not allowd to cast their net widely enough to possibly include union busting.