This is actually quite an interesting question because Tom is MEANT to be an ambiguous character. Phillips himself says, Tom is an "ambiguous creation, a carelessly sensual jock with an insecure streak."
I guess I would say that the simplest answer to your question is that Tom's attitude toward Beth is indifference. In the simplest sense, Tom is Beth's husband (and soon-to-be ex-husband). Tom has cheated on Beth (who has her own issues, trust me) which is reminiscent of his philandering youth, but as he tries to explain his side of things while seeming distraught, he casually eats snacks and switches subjects. Tom seems even more shallow when we see him initiate sex in the middle of a huge fight with Beth AFTER he has cheated on her!
There are other insights into Tom's character during flashbacks. Earlier in their relationship, Tom accuses both Karen and Gabe of "setting him up" with Beth. Perhaps the relationship was all a "scheme" of Karen and Gabe. Tom's thoughts on the subject are as follows:
That's okay with me. I have no problem with cheap and scheming.
If we take Tom at his word, then it is no surprise that he would regard his wife, Beth, with indifference. Ironically, Tom endeavors to take that statement back when he considers his "new woman" that he has left Beth for. My favorite idea behind the feeling of indifference is explained nicely by eNotes:
[Tom] grew up with parents who stayed together not because they were in love, but because they wanted to keep up the appearance that they were a happy family. For the sake of the children was a popular phrase that referred to the fact that a married couple would stay together, even if they were miserable, because they thought it was best for their children. Tom doesn't want to repeat his parents' mistakes.
Tom wants to live a life that is "authentic." Of course, this doesn't include Beth. How ironic that being "authentic" involves nixing an earlier commitment. Beth matters not. Tom wants NOT to be his parents. Tom knows that his parents simply stayed together while not remaining in love. They did it "for the children." According to Tom's new self, this is not "authentic." Therefore, he needs to leave Beth, as he is no longer in love with her.