When Richard asks Therese "Do you like caviar?" and she says, "No. I wish I did" (14.84-14.85), their exchange is reminiscent of a scandalous conversation from the 1960 film Spartacus in which the General Crassus says he likes eating snails and oysters to explain, via innuendo, that he is bisexual.
In this conversation, Richard is the caviar—and Therese doesn't have a taste for him. It's not exactly his fault. It's just that he happens to be male, and Therese is starting to realize that she is attracted to women instead.
Therese makes a point to mention Richard's feminine girly soft hands, and it seems like that might be a reason she dates him. He's not one hundred percent caviar, you know? But later, Therese talks about Carol's hands, too, saying, "Carol's hands were strong" (12.90). So that rules out the hand theory. Sexuality and gender are on a spectrum, and Therese likes people who are mostly feminine and a little masculine—Richard is the other way around.
So why does Therese like Richard? She doesn't want to hang out with him. She doesn't think he's a good artist, either, and she pictures him as a lumberjack instead. If only Richard lived in present-day Brooklyn, where all the artists look like lumberjacks. Therese says she likes him because "He does treat me like a person instead of just a girl he can go so far with or not" (7.73). Despite this assessment of him, Richard still says things to her like, "You haven't enough respect for technique, little girl" (5.22), which is insulting and makes us wonder what sorts of things other men have said to Therese. If Richard looks good, we don't want to meet the dudes who look bad.
Some people would probably be good friends if they didn't bring romance into it, but Richard and Therese seem to be together only because of societal pressures to get married. We don't think they'd spend any time together otherwise.
Societal pressure is why Carol married Harge, and Richard has a lot in common with Harge. Therese is lucky to live in a more modern time than when Carol got married, but she's still expected to get married—she just feels like she has the freedom to explore other things, too.
Carol doesn't think Harge actually loves her, only that he's trying to control her, and as the book progresses, it seems that Richard is the same way with Therese. He says he loves her, but he gets increasingly angry with her. He thinks he can change her, saying, "This won't last, Terry. I knew a little about such things" (16.101), and he says she makes him feel "Disgust" (21.7). Oh good.
Carol is actually the one to have sympathy for Richard, recognizing he's in a difficult situation, but we don't see Richard and Therese making up any time soon. Dannie thinks Richard acts out because "His ego's suffering" (22.104), but whatever the reason for his behavior, Therese is thoroughly disenchanted with him.