Some theories
Here are my thoughts
* Katherine's husband had cheated on her before. He had sex primarily with students. He had perfected his ''cheating'' though in that he mostly did it with young women he taught seminars to in the U.S. My guess he did it less while in Toronto Canada and it was harder for her to catch him in the act. Meeting Chloe he seemed very guarded and he was unlikely to sleep with her as I said because he did it with women in New York where he couldn't be found out (no one around that they knew to catch him).
* Chloe did not see Katherine as a mother figure or lover. She was jealous of her and her way of living. She wanted ''the life'' Katherine had. She had a desire to actually RUIN Katherine's life.
* Chloe infected Katherine's son with HIV to hurt Katherine. It's never determined in the film but its slightly hinted. Her ''clean bill of health'' was faked. Chloe had HIV.
* Though Katherine's husband was cheating on her with other girls and women I do think by the end they reconnected emotionally. He had fallen out of love with her before and then back in love with her. I think he may no longer cheat... but there's also a chance he's a sex addict too so it's unclear how it all ended up.