...would undermine the Left's attempts to use trans-people for political gain.
They know that forcing the right to ignore biological science is divisive (you must say "she" even when it is a male), and group divisions like that motivate their base. They paint science-support as bigotry, which motivates low information liberals to vote...their personal feelings after voting is more important than scientific truth.
Solving this problem takes away a voting issue for them.