To me, Bostock v. Clayton Cty. clearly resolves this issue. In Bostock, SCOTUS held that Title VII's prohibition of sex discrimination also prohibits discrimination because of an employee's sexual orientation or gender identity. Indeed, Bostock could not have been clearer regarding the treatment of transgender employees: "[a]n individual's … transgender status is not relevant to employment decisions."
While Bostock focused on employment terminations, Title VII also prohibits discrimination regarding "compensation, terms, conditions, or privileges of employment," including employer-provided benefits.
Thus, under Bostock, if an employer discriminates against transgender employees in the provision of employer-sponsored health benefits, that employer violates Title VII.
Two other federal courts (in Ga. and N.C.) have already reviewed this issue and agree. For example, in Lange v. Houston Cty. the M.D. of Ga. held: "Transgender employees cannot get medically necessary treatment for 'sex change' medical care because they are transgender. On these undisputed facts, the implication of Bostock is clear." I suspect that Morgan Mes's lawsuit will reach a similar result.
Even though SCOTUS seemed to definitively resolve the issue of workplace transgender rights, this issue continues to be controversial. It shouldn't be. The law is clear. If you are considering denying insurance coverage for gender-affirming care to your employees, you are taking a huge risk. You don't have to agree with or support these employees' gender, but you do have to follow the law.