In most cases it is probably unwise for partners in a loving relationship to go outside the relationship for further sexual gratification. There are a number of reasons for this:
Adultery is usually carried out in secret. This leads to feelings of guilt and anxiety on the part of the unfaithful partner.
Trust is an essential ingredient in most relationships. Adultery, once discovered, or even suspected, destroys this trust.
If one partner is getting all the sex they need from outside the relationship, the other partner may be deprived of any sex.
Adultery takes time and sometimes money that would be better spent on the marriage.
There is always the risk of disease or pregnancy to be considered.
One of the main reasons that it is unwise is because most people within our culture hold irrational beliefs about their relationships and become jealous if they discover, or even suspect that their partner has strayed. This places an enormous and sometimes fatal strain on a relationship. Jealousy usually stems from one or both of the following beliefs:
I have exclusive ownership rights to my partner, or
My partner's paramour is better than me
In some circumstances, if neither partner holds the above beliefs, then extra-marital activity may be harmless or even beneficial to a relationship. Let's look at some reasons why adultery might be beneficial:
Adultery provides variety that helps to stimulate the sex drive. If this increased arousal is brought back to the marriage bed it can enhance sexual relations between the couple.
Adultery is an opportunity for sexual learning, which again can be brought back to the marriage bed.
An adulterer's remorse can stimulate a more loving approach to his or her partner.
In a relationship that is good apart from the sex, adultery can help to save the marriage by providing the partners with what is missing form an otherwise healthy relationship.
Some couples benefit from adultery but most don't. Given that the result is not always the same in every case, I can't see how we can automatically say that it is good or it is bad. All we can say is that "It depends..."