Tiger was a player before his marriage, and the vows he took didn't seem to matter or curb his behavior. That was his choice, and now he's feeling the consequences.
I think Elin might have stuck it out if there had been only 1 or possibly 2 women, but when it gets up into double digits how could any self-respecting woman just overlook that? That and the knowledge from one of them, Grubbs, that he didn't use condoms. That alone should make the wife furious, especially since he was allegedly conducting his affairs while Elin had their two children, thus putting them all at risk.