My answer is that Anthony is a master manipulator. He is passive-aggressive most of the time and aggressive when he needs to be. He feels inadequate, but also has a high standard of the type of woman he thinks he deserves. So...I postulate that Anthony treated Therese like he treated Liz during their courtship. He treated Therese like she was so far above him, mooned over her, thought things at her, showed up where she was by being in the same circle of friends she was in, liked the same things she liked such as ballroom dancing, etc. Ironically, he may have learned this sort of behavior from Liz herself. Didn't she do this sort of thing in every relationship she had? Liz tended to make herself over into someone her potential boyfriend would like. Anthony may have done the same thing. He also loves his rescue fantasies. Maybe he "rescued" Therese in a way...it doesn't have to be a huge thing like Liz's assault, but something that Therese would appreciate, like giving her a lift somewhere, or picking up fallen notes and books for her.
Therese would have known Anthony for a while from sight, since he was friends with a few friends of hers. Maybe they were all in accounting classes together. She would have been glad for a ballroom dance partner since women tend to outnumber men at these things. She may or may not have been flattered by how Anthony was always looking at her with soulful eyes. Some women would have been skeeved, but let's say Therese wasn't, because she ended up loving and marrying the guy. I would also say that Therese wasn't very experienced in relationships. Maybe she went to an all-girls Catholic school? I don't think she was nerdy or socially awkward, since she has no problem being in high management positions, dresses well, speaks well, and so on. Anthony would have been around more and more, until one day he suddenly blurts out at Therese that he can't stand it anymore, he had to try for a date with her. He had been wanting to for months and months. Therese would have been flattered. Anthony may have even said that she was the first woman he had felt anything for since he and his ex-girlfriend broke up. Just like Liz, Therese would have felt as if she was the heroine of a romance novel, that she was The True Love.
I'm still pondering on whether Anthony cold-bloodedly picked Therese to be his perfect woman to make him forget Liz, or he subconsciously does these things.
As for his future with Liz...I think he will start searching for another woman to be his perfect one when Liz starts making him miserable, by ballooning up in pregnancy and no longer being pretty, being irritable, putting unreasonable demands on him to do housework since she's really a slob and is always screaming at Francoise all the time, even as he manipulates Francoise in doing what Liz wants just to shut her up.
Let me know what you think!