It's not ridiculous at all.

The coach originally prayed with students then stopped when the school asked him to. He prayed alone while off the clock during his firing. That's not analogies at all to your scenario about teachers inviting students to make out.

The dissent wasn't that he was overtly pressuring students to join him in prayer on the field. It was that his position of authority made it so merely knowing he was religious would be enough to pressure students. Teachers are in positions of authority over students too. Heck, plenty of places have specific laws that make it a crime for a teacher to date an 18 year old student because it's a position of authority. So shouldn't it stand that if a teacher being religious is enough to compel students into religious that a teacher being gay is enough to compel students into homosexuality?

The only real difference here is one is religious and the other homosexual. That you're applying wildly different standards to the same scenario says a lot.