Bed egoism: how to understand that only his orgasm is important for a partner?

Sex should be a pleasure for both, but if one of the partners is only interested in himself, the second is usually unpleasant. One cannot call egoism the desire to get one's orgasm, but when a person refuses it to another - is it good? In addition, sexual selfishness can affect other aspects of a relationship. Here's how to understand that a partner thinks only of himself.