Many couples and cultural norms find sexual contact outside of a relationship wrong and distasteful. The actual rules of a relationship are up to the individuals in it, not what rules or conventions tell them they should be.
In fact, most would be surprised to know just how many couples experience sexual contact outside of their primary relationship.
There are many reasons why couples would consider additional sexual partners for satisfaction. A heterosexual couple may have homosexual desires, one partner may have a fetish that the other is not able to or comfortable in fulfilling, or they simply want to experience other sexual styles that they have not been exposed to. In any case, the why need only be apparent to the individuals in the relationship.
Couples who engage in sexual contact outside of their primary relationship may have a variety of rules in place to keep the contact in a specific context. Some couples may opt to exclude certain forms of intimacy, such as kissing or having the same extra-marital partner more than one time. Rules help to keep both partners in the relationship feeling safe in the activities they pursue. Those who create unreasonable rules for their partner may be showing they are not as comfortable with the idea as they had originally communicated. If this happens, discuss how realistic the plan is before continuing on with logistics.
In many relationships that include sex outside of the relationship, a rule exists where the partners do not discuss the exploits of the other outside of the relationship. In this case, there must be a high level of trust that your partner will not bring home disease or lice infestation. Not discussing additional partners helps some who have jealous tendencies, but still wish to explore another side of themselves.
Finding sexual partners that are outside of your relationship may not be something that you can advertise within your group of friends. Consider placing personal ads if you are seeking a specific fetish that cannot be satisfied with your primary partner. In some cases, part of the bargain between couples is that neither will actively seek out others. The sex has to come to them for the terms to be held. Sex outside of your relationship is only considered cheating when your current partner is unaware and has not given their consent to your encounters.
Sex outside of any relationship should have rules that will help to guide the partners. Keep true to your rules and there should be no guilt in finding your bliss.