Eh, to be fair, said English boarding school had both sexes as students, and aside from that, there were plenty of professors of both sexes. Besides, I’m pretty sure heads of Catholic boarding schools in England aren’t gay (or at least, they’re not SUPPOSED to be that).
And honestly, I really didn’t see Dumbledore as being gay at all until JK Rowling dropped that bombshell. Maybe if he behaved like General Blue from Dragon Ball when he encountered Bulma here: https://www.youtube.com/watch?v=vLfcQgJ47cU or heck, how Otokosuki acted towards Trunks in Dragon Ball Z here: https://www.youtube.com/watch?v=IBTQrNeg7Ow I might at least get some sense he was gay. But as it is? He came across as at most asexual.
It was no bombshell. It's right in the open in the last book.