One sort of extramarital affair revolves around sexual addiction. The partner drawn in the affair, plain and simple, has a hard time saying "NO." He or she may want to, but feels obliged to say "yes."

People can’t say no? Well, I think all people have the capacity, at some level, to say no. Though, not all have developed that capacity or reached that level to firmly say no and mean it.

Some individuals seem to lack the capacity to constantly act on the "no." Don’t forget that all of us are "grabbed" by something and find it hard to let go. Infidelity when associated with sexual addiction and its many forms, though, becomes an influential focal point.

How to find out if infidelity is attached to sexual addiction:
 Sex takes on an exaggerated role or value. Sex, sexual conquest, sexual release turns out to be an influential power. Acting on the sexual impulse is a regular doings. Thinking about sex similarly consumes an excessive amount of time. Manifold ways of acting out sexually (porn, strip clubs, multiple sex partners, etc.) are ordinary. 
 A promise/failure cycle recedes and gets back with the inability to say no. After an "acting out" episode the person usually experiences guilt or fear and promises to self or others, "I won’t do it again." This will last...until the "urge" is acted upon again. 
 This activity is limited by dread. The person lives with different fears, namely: the fear of getting caught, the fear of outcomes, the fear of "being found out," the fear of being nonstandard, the fear of being punished, and the fear of losing family, spouse, work and respect. 
 Others are used or seen as objects for individual satisfaction. No true intimacy is developed. 
 Sexuality is often puzzled with other needs or associated with untreated past pain or shock. A child who experiences embarrassment around sexuality or sexual abuse of one form or another, may carry along that confusion and effort to "work that through" in a marriage or extramarital affairs. 
 Such a person lives in a twisted world. They see the world and relationship through the eyes of their "addiction." They have a great ability to improve their behavior, mislead others and may lead a "dual" life.

Note: If you think these characteristics fit you or someone you love, seek some help before your world disintegrates further or falls apart.