@BobcatNo6400

She finds a guy better than you, she cheats. It doesn't get any simpler than that.

1) it can also be a guy she only perceives as better in some way. More exciting, etc.

2) some women actually have decent character and won't even be alone with other dudes at all or put themselves into tempting situations. Rare, but they exist.

I'm not afraid of losing her

Good.

From Roissy's Sixteen Commandments of Poon we have number 16:

XVI. Never be afraid to lose her

You must not fear. Fear is the love-killer. Fear is the ego-triumph that brings abject loneliness. You will face your fear. You will permit it to pass over and through you. And when your ego-fear is gone you will turn and face your lover, and only your heart will remain. You will walk away from her when she has violated your integrity, and you will let her walk when her heart is closed to you. She who can destroy you, controls you. Don’t give her that power over yourself. Love yourself before you love her.

Just as a side note, the dude you're replying to needs to stop expecting other users to spoon-feed him and start reading and thinking for himself.