Having an affair means having sex with someone while you are in a steady relationship.
Some couples promise each other they’ll only have sex together and not share these intimacies with anyone else. Others never talk about this openly, but just assume the partner won’t mess around with others.
A minority of the couples decide to have an ‘open relationship,’ meaning it’s okay to sleep around a little bit.
If you find out your lover is having an affair it’s most painful when you assumed him or her to be monogamous and unexpectedly find out this is not the case. You feel betrayed and your trust may be severely damaged.

How monogamous are Dutch teenagers?

I found out my boyfriend is having an affair. What should I do?
How can I tell my boyfriend I have cheated on him?