I'll describe a scenario.
In your opinion, what made the wife cheat?

It goes like this:

  1. A fairly alpha guy gets a good looking girl.
    They are in a LTR for some years.
  2. That guy has a best friend - childhood friend.
  3. The guy knows how to treat its girl, meaning like alpha (at least in public, don't know what happens when they're at home alone)
  4. Some months before the wedding, the girl cheats with his childhood friend.
    It keeps going also after the wedding.
  5. Finally, some weeks after the wedding the childhood friend tells the guy about the cheating.
  6. Relationship ends forever.

​

Edit:

By writing "alpha guy" I didn't mean 100% alpha, but he definitely has alpha tendencies and behaviors, though it seems that he's unaware to RP.
See my comment to "Unlikely_Composer" for details.