Montgomery County, MD--one of the collar counties of Washington DC and among the most liberal in the US.
When I was in the 10th Grade, opting out meant you when to the library and did a short research paper on eating healthy or exercise, while the sex ed stuff your parents didn't want you to participate in occurred in the classroom.
There is no reason the school cannot supply alternative work units for students whose parents don't want the school teaching their children about sex.
There is no reason to not allow graduation of students whose parents don't want them learning about homosexuality. Unless your agenda is to force that perspective on students that are not old enough to make such decisions yet.