According to the Politico, US House of Representative members Rep. Steve King of Iowa, Rep. Mark Meadows, and Rep. Diane Black along with 37 other conservative house members sent a letter today (Friday, November 16, 2018) to the Trump administration expressing their concerns of the LGBT protections in the USCMA agreement. The letter indicates that the LGBT protections may cause some of the signatories to the letter to not support the agreement.
In short, chapter 23 of the USMCA has requirements that workers be protected from discrimination on the basis of sex, including sexual orientation and gender identity. These LGBT protections were a priority for Canada and the Trudeau government.
The US, Canada and Mexico are expected to sign the USMCA at the G20 summit on November 30th in Argentina.