Is it fair to have a lover outside marriage?

I would imagine if you had an "open" marriage, then yes alls fair in love matters.

Marianne Ginther says no.

What were your vows when you were married ?

You use the words 'outside marriage' which implies it is a secret (else it would be 'another lover inside marriage')

It's not OK to have secrets from your partner that could affect the marriage. It is the dishonesty that is destructive.