However, cutting to the chase, would somebody please explain what damage would be done , and to whom, by a 16 year old boy shagging a 22 year old female teacher (using appropriate precautions, natch)

You don't need to explain the damage done by a 22 year old male teacher shagging a 16 year old girl ... - Murray

I don't think there would be as much of a difference, boy or girl (other than pregnancy risk, but that goes both ways too), as you seem to suppose. There certainly would be some sixteen year olds not harmed at all, and of course, a lot would depend on the teacher's personality.

Most teens that age, though, can still be pretty uncertain about themselves, exploring relationships, sexuality, dealing with self esteem problems, and so on. Throwing a mature authority figure into the mix doesn't strike me as a good idea, not to mention, what 22 year old is so attracted to a sixteen year old student that they take such risks? Smart enough to be a teacher but not smart enough to hold off until the kid's at least out of school?

And what is the teen's motivation? Are they really attracted, 'in love', or are they feeling intimidated about it, going along because they don't want to be rejected or even treated punitively by the teacher?

Too many risks, IMO, to be encouraged in any way.