A core issue when discussing the topic of fidelity and monogamy is trust. For many people, it can be difficult to fully trust someone else. This is especially true in cases where the person has previously been hurt or betrayed by a lover or someone else he or she trusted. So it can take a lot of time, patience, and effort to establish a trusting relationship.

As far as sex goes, while casual sex can be no sweat, it generally takes a lot more work to establish a committed sexual relationship based on a foundation of trust. This requires both parties to break down the walls they may have put up around their emotional core. They must allow themselves to be open and vulnerable, which can be scary — if not downright terrifying — for many people.

With a committed sexual relationship, there are two issues in play. First, you are opening yourself up to your partner emotionally and trusting that person to protect your emotional well-being. But you are also opening yourself up sexually, perhaps revealing your most secret sexual thoughts and desires. You might eventually feel comfortable enough to reveal thoughts or fantasies you have not shared with many other people — or perhaps have never shared with anyone before now. You may also work up the nerve to engage in sexual activities you were afraid to try before. This can leave you feeling exposed and vulnerable, and force you to rely on the trust you have in your partner to guard those secrets.

For many people, this is the basis behind the desire for monogamy. Reaching this point of trust and vulnerability with another person is a huge accomplishment, one that does not come easily. When you have achieved this state, you share a powerful intimate bond, one that can be tough or impossible to replicate with another person.

If this is your view of your relationship, it can be devastating to imagine that your partner has disregarded this safe and trusting state that you worked so hard to achieve. To you, it is not just the point of a physical betrayal — your partner has also crushed your trust and possibly destroyed the foundation of the relationship. Your partner has made you regret allowing yourself to become open and vulnerable, which can be a huge setback to overcome.

Of course, there is the added issue of your partner's disregard for your physical well-being, if he or she had sex with another person and did not take proper precautions, thereby leaving you at risk for various sexually transmitted diseases.