In titis, one of the many small monogamous South American monkeys, females are very intolerant of the approach of strange females and will drive them away. And I have observed similar behaviour in a small monogamous African antelope known as the klipspringer. Clear off Maria Sandell of Lund University, studied this experimentally in European starlings. During the egg-laying period, strange females were placed in small cages near to the nest box used by an established wild pair. Males offered the opportunity of a second female showed considerable interest, but their females were rather more aggressive towards the rival.
Any monogamous partnership is built on tension.
Flexible behaviour allows individuals to exploit every opportunity to pursue their needs. So we should not be surprised to see that nature is full of partnerships being dissolved as new and better opportunities come along. Estimates of pair-bond dissolution vary enormously, both across species and, within species, across populations. Over half of all pairs of Belgian great tits, for example, get divorced. Failure to rear offspring is one common cause of avian divorce.