I’m actually sympathetic to the notion that disapproval of infidelity makes no sense when it is detached from a robustly conservative view of marriage and sexual morality. But Dawkins doesn’t deal at all with the question of infidelity as a breach of trust, which causes at least as much hurt to the betrayed party as jealousy. Presumably he would say that people should not ask for promises, even implied promises, of fidelity. But given that they have gotten them, shouldn’t they be angry at their being broken?