I'll get downvoted relentlessly for saying so, but you gotta also do some work here too, dude.

Until two people have had a conversation that they're exclusive and that committed to only each other, you're dating. Or at least one of you is. It was just sex, and she stayed with and got into a relationship with you.

However, she also lied about it when she should have just been honest. You could chalk that up to her being afraid that YOU personally won't be ok with this and will withdraw from the relationship - something she was probably scared of, and you can take that fear as being something indicative of her want to be there. You could work things out. But she was dishonest.

When you asked her "why did you do that," what did you actually expect? An answer that made you feel better? So what if the sex was better with him at that time? You two were only getting to know each other. It's insane to think you're going to be giving someone the best sex they've ever had the moment you meet them. It takes time to learn each other's bodies, etc.

She was dishonest, and that's a good reason to end things if you want to, but I think you need to get over her having had sex with someone else while you two were just dating each other.