Is it normal for husband to go to his ex-wife's house for dinner with people in their twenties every day?

Nothing unusual. Even if there are children of the same sex after divorce, the children have grown up, and it is not a strange reaction to pay attention to them. In fact, such things are hard to put an end to. The more you care, the more you want to interfere. In fact, it will only lead to the alienation of your relationship. There are two kinds of feelings in the relationship between husband and wife, parents and children. Therefore, your relationship is only the third, but not the first.