In the livestream, he said that he wasn't having an affair-- he did not want a relationship, he wanted sex.
That has always been his approach. Sexting with "women," his obsession with jerking off, paying for sex, etc. He likes the physical thrill, but does not want any strings attached. A wife and two daughters are a lot of strings. The "problems with their marriage" and their "incompatibility" with one another might simply be a case of him wanting her to be there when he wanted sex, and staying out of his way the rest of the time.