Female superb starlings cheat. But they cheat for a good reason: they trade sex for the sake of their chicks!
Some females mate with subordinate males from within their social group when they need help to raise their chicks. (Superb starlings are cooperative breeders, meaning breeding pairs get help in raising chicks from other family group members.) This additional male then also acquires food and tends to the nestlings, which increases the chicks' survival rates. [...]
... some females cheat with males outside their group if they sense their mates are too genetically similar to themselves. Mating with strangers increases their brood's genetic diversity, even though it does not reap additional help.