My Fiancee Is Sleeping With My Friends But I Still Love Her

I’m 31, she’s 26. We’ve been living together four years and are planning to marry and have kids.
I have always trusted her totally and never tried to stop her going out with her friends.
I have my night out with my friends too and never thought either of us would cheat on the other.
Then some of my friends started acting weird around me. I asked another friend what was wrong. He said he had no idea but looked very awkward and I could tell that he knew something.
In the end a couple of months ago another friend took me aside and said he didn’t want to hurt me but he thought I had a right to know.
He told me that my fiancée has had sex with at least three other men since she has been living with me. He told me things they’d said which they would never had known if it wasn’t true.
I confronted her and she just passed it all off as lies and jealousy, but there’s no way it can be.
I still love her, even though I know what she’s done. I keep having visions in my head of her cheating on me and it makes me feel really sick.
I want to give the relationship another go but she won’t talk about it. She just denies everything and that leaves me with loads of unanswered questions.
I want to move this forward without anyone getting hurt.
What do I do?