I slept with my ex's best friend and I don't think he knows about it.
I hope you're not serious! Why would you sleep with your ex's best friend?
Loyalty was never my strong point. I wanted to have sex with him. So, I did it.
It's inappropriate to sleep with your ex's best friend because it could ruin their friendship. I think you should forget about your ex and move on with your life.
It's not a big deal.
It may not be a big deal to you, but if your ex finds out, they will almost certainly be very mad at their friend. By sleeping with him, you are putting the friend in a very awkward spot where he may have to choose between you and your ex. You should tell your ex's friend that you can't sleep together anymore.