She didn't want to go without her partner.
The whole thing seems like a TERF exercise, that's my main problem with it. BTW, the TERF's wouldn't want an MtF at the dance either, as we only exist to promote rape culture from within the ranks of women.
I think it's just bs to take a pair of women - consider them members of the community, and then wait for one of them to go through this incredibly hard transition, and then kick them to the curb.