It would be one thing if it was just a younger man, even one who is 17/underage in America. For me the bigger issue is how it and Michelle have been played: As a desperate, slightly weird/creepy middle-aged frustrated hausfrau who everyone calls stuck-up. It's just not the Michelle I remember in any way, except for her occasional scene with the family talking about the good old days or dispensing some home truths. And even then she is very placid. The character as written and performed is literally the classic stereotype we have in America of desperate, unhinged lonely women who seduce young boys in school, or marry convicts in prison.
If Michelle had come in blunt, tough and told Sharon the truth without doing some sort of sub-Tennessee Williams monologue about cellulite and so forth - said she screwed up and slept with a student who'd made the first move, that it was good and that she knew it was wrong - that would be a horse of a slightly different color for me.