Infidelity isn’t necessarily sex addiction. Sex addiction, like other addictions, is a dependency on something that stimulates the pleasure center in the brain, and fairly quickly, the brain wants more, setting up a dependency pattern.
Whereas infidelity is often one relationship outside the bonds of another committed relationship, usually marriage. Infidelity, such as an affair, is quite often only one indiscretion, with no ongoing pattern of stepping outside one’s committed relationship.
Even so, many sex addicts do have ongoing affairs, often with multiple people. So while an affair can be one, isolated relationship, so too can an affair be a signal that other forms of non-monogamous sexual activity may be going on outside the marriage.