We often hear of kings and queens having "mistresses" (or whatever the male equivalent is) and it never seems to be treated as a scandalous thing in history books as well as in films and literature. Was it just a fact of life that monarchs were allowed or even expected to have extramarital affairs?
Yes, but I would specify and say that while there were some exceptions, this really applied to men, and Kings. That doesn't mean that everyone even liked it when Kings had affairs, but their reputations generally wouldn't be at stake like a Queen's would be if she had an extramarital affair.