I have read some academic papers that include it in romance addiction, love addiction, or sex addiction, depending on what the fix is for the infidelity...
But infidelity itself no, its not often viewed as a disorder...
Pursuing romance, sex, or love even with your spouse in a manner that is destructive is categorised as an addiction or impulse control disorder. WHOM you are with isn't the issue.
I think infidelity sets the stage quite well for love, sex, or romance addictions though... it pretty much wraps the addiction in a big red bow with a card and some chocolates and says "here, want an addiction... start lying, sneaking around, and hurting your wife and family... Its very addictive and you will have LOTS of fun"
