In the age of birth control and casual sex, is sexual jealousy and monogamy in relationships something that has outlived its usefulness?
Should people disregard sexual fidelity?
It really depends on the people in the particular relationship. If a couple agrees to have an "open" relationship, good for them. If they have no problem with their partner sleeping around, fine. But this must be a two-sided agreement, and not one partner being selfish at the expense of the other.
Sexual fidelity, as with any type of loyalty, is an indicator of the strength of a relationship. But to be faithful doesn't necessarily mean having no partners except your life partner. For some people, it might just mean being honest about sleeping with other people.
Obviously, though, the proportion of people who are actually able to feel no jealousy, envy or other bad emotions about a sexually "unfaithful" partner are in the minority of the population.
Sexual infidelity, in which one partner has other relationships without the other partner's knowledge, is a sign of dishonesty in the relationship in general. And once trust is gone in a relationship, things become much harder, I think.