The essential problem is of material support.
Reiichi doesn't seem ever to have romantically loved Kaoru, but he saw that she needed someone to provide for her materially, so he married her. Now he may have developed romantic feelings for another woman.
Kaoru has thought things that hint that Uta's feelings may actually be reciprocated.
But Uta is a high-school student, and Kaoru is a housewife. They're not positioned to establish their own household.