So you caught your boyfriend cheating with his ex girlfriend and to get even; your plan is to cheat with his best friend.  What a screwed up situation.  A wrong for a wrong never fixed anything.  A relationship is not all about revenge and getting back at the person who hurt you in a reciprocal manner.

Your natural instinct is to get him back for disrespecting you and your first inclination is to throw him some of his same tactics.  Won't work.  What a mess to call such a situation a relationship.  The relationship was over when you caught him cheating.  If you had been holding him up on a pedestal with your love and he cheats with his ex, that means that he just kicked himself to level zero.  You do not have to play games and then think that you are in any type of relationship because you continue to go out together.

The fact is that unless he is a talker and most guys aren't, then you can get to an explanation;

 hopefully an apology and to move on with the relationship.  The boyfriend's loyalty to a relationship however, remains questionable and it is up to you to believe or disbelieve his new contentions of being true to you.  The boyfriend's decision to play the field so to speak while in a relationship with you indicates however that you were not his top priorities.

What could have been more important than saying no to the ex and thus proving to himself that he was truly in love with you and that you were the one.  The boyfriend has to be true to himself before he can be true to anyone else.  At that very moment when he gave in to his ex meant that he valued his infidelity more than sustaining a relationship with you.

Sex to most guys aren't as important when it comes to being in love.  Some guys can use the word flippantly as a good morning greeting.  These are the ones who really need to prove that only to their girlfriends but mostly to themselves that they can be true.  Loyalty in a relationship seals the love and commitment in the relationship for endurance and for an everlasting bond.  Most guys do not like the solidarity of a relationship and want out of the enduring and lasting commitment relationship.  It has been remarked that most guys are afraid of the word. "relationship." These are the guys who are likely to cheat on their girlfriends with their ex or whomever they find attractive and willing.

If you have on the top shelf of your love cabinet the provisions of loyalty and faithfulness in the relationship and you find that your boyfriend has reneged; you should make the proper placement of your love.  Your love for him should not remain on the top shelf alone while he have his fun and expect you to be there when the ex have moved on.  Unless you think of yourself as a doormat and everything that you own emotionally as subservient to him, then you should move on as well.

Do not cheat with a cheater because all you would have done is to put yourself on the same level on the down-low.  Keep your emotions that are so much a part of you available to someone who will be loyal and will not venture out with every cat call.  A cheater cheats for selfish reasons and as you are not in his deepest confidence when he is faced with infidelity, find someone who is more worthy.  Your loyalty and faithfulness in a relationship is worth far more than paying a cheater back by being a cheat yourself. 

 You are not a cheater and should not change your facade as a reaction to the dishonest deed of your boyfriend.  You should not cheat because your boyfriend cheated to get back at him.  Such a detrimental action would result in lowing yourself to the standards of the dog pound of which your boyfriend would have made his home.  Let him enjoy his new home of disloyalty, dishonesty, noncommittal, untrustworthiness and unfaithfulness alone and without you.  You should end the relationship with the cheater  and move on