"Whether the prohibition in Title VII of the Civil Rights Act of 1964 against employment discrimination 'because of . . . sex' encompasses discrimination based on an individual's sexual orientation."
Why not grant certiorari? It could be (Hey, Just a guess) the awkward posture of the case. The Respondent had never been served with summons and complaint. This was because the pro se plaintiff was proceeding in forma pauperis, which allows a magistrate judge to screen out and dismiss the case if it fails to state a claim on which relief may be granted. That's what happened, so the employer never was a party in the case. Once the Supreme Court "requested" a response, the response was:
"The named respondents take no position on whether this Court should grant the petition for a writ of certiorari. If certiorari is granted, they do not intend to participate in the case." [Brief]
The Court often denies certiorari when a case is not a good "vehicle," meaning that it's messed up with side issues that the Court doesn't want to deal with. Could be that happened here. Could be they just want to hold out while some more Circuit Courts weigh in on the main issue. Could be any number of things. We'll never know.
Jameka Evans claimed that her supervisors harassed her and otherwise punished her because of her sexual orientation. The district court magistrate judge said Title VII “was not intended to cover discrimination against homosexuals.” The district court dismissed the case, and the 11th Circuit held that Evans could not state a claim by alleging workplace discrimination because of her sexual orientation. Evans v. Georgia Regional Hospital (11th Cir 03/10/2017) [Opinion].
There is a split of authority between Circuits. Hively v. Ivy Tech Community College, 853 F.3d 339 (7th Cir 2017) (en banc) [Opinion] held that – indeed – Title VII forbids discrimination on the basis of sexual orientation. The 7th Circuit – typically described as a "conservative" court – voted 8-3 on this issue. Also, two judges (out of three) in Christiansen v. Omnicom Grp., Inc., 852 F.3d 195 (2nd Cir 2017) [Opinion] expressed the opinion that Title VII forbids discrimination on the basis of sexual orientation; however that court held just the opposite because they lacked authority to overrule prior circuit precedent. Zarda v. Altitude Express, Inc., 855 F.3d 76 (2nd Cir 2017) [Opinion] also asserted a lack of authority to overrule prior circuit precedent, but that case is now under en banc review by the entire 2nd Circuit, which heard oral arguments on September 26.
At the same time, there are plenty of court decisions taking essentially the same position as the 11th Circuit's.