Employers that discriminate against employees on the basis of sexual orientation or gender identity are playing a dangerous game. Many states, unlike Pennsylvania, prohibit such discrimination. Here in Pennsylvania, we do have municipalities with anti-discrimination ordinances that cover sexual orientation and gender identity; and, state employee and state contractors will be covered by executive orders.
On the federal level, you have the EEOC's argument that Title VII's prohibition of sex discrimination already covers sexual orientation and gender identity. Throw in some courts protecting employees under a "gender stereotyping" theory that sounds an awful lot like protection for sexual orientation; and courts protecting transgender employees under Title VII.
Sure, employers can still make the argument that "sexual orientation" and "gender identity" are not expressly covered under Pennsylvania's state law (PHRA) or Title VII... but it's not clear how far that will get them.