I guess I disagree a little with you because my line was when he cheated on Elin and put his kids through it the first time. Personally, I see that as involving another family (Elin and the people that vouched for Tiger to her and the kids).
What I don't get is why? Nothing required him to get married in the first place. He could've remained single, avoided sleeping with co-worker's wives (and ex-wives), and nobody would have cared.
Instead, he cheats on his wife repeatedly, and then sleeps with his co-worker's ex-wife. Amanda Dufner/Boyd is definitely hot, but she's not that hot, and he's Tiger and could probably pick up just about any woman he wanted. So, what the heck?
I agree on the family part & while that wasn't right either and it arguably did more damage to the kids, it was contained within his "family".... This latest allegation is him tearing up another family independent of his own.
Trust me I don't live in a glass house & im not on my moral high horse, but I have a line & it was crossed in my eyes. Inflicting damage to yourself or being destructive to your own situation is one thing, but doing it to another family, arguably destroying that relationship in the manner in which these allegations detail is a whole other transgression IMO.
I also agree he could get anyone he wanted so why her? Knowing it's a fellow tour players wife (however rocky that relationship was). She's not without fault either, but this show more of TW's ego and the false sense of reality he lives in.