infidelity

uskottomuus unfaithfulness Unfaithfulness in a marriage or an intimate relationship: practice or instance of having a sexual or romantic affair with someone other than one's spouse, without the consent of the spouse.
2013, William G. Staples, Everyday Surveillance: Vigilance and Visibility (ISBN 1442226293), page 155:

Your friends tell you rumors about your girlfriend's infidelity or you remember being broken up around the time the baby was conceived.

It was disastrous that England's infidelity towards Frederick the Great — which no one, not even a German, condemned more strongly than did William Pitt — had to affect one of the most popular heroes of our national history.

The means used to this purpose are partly didactical, and partly protreptical; demonstrating the truth of the gospel, and then urging the professors of those truths to be stedfast(SI) in the faith, and to beware of infidelity.