Why Wont My Boyfriend Just Tell Me The Truth?

Okay My Boyfriend sweared to me that he has never had sex and ive heard lot of things of him from people that he has had threesomes with girls and my own best friend told me she had sex with him and another guy at the sametime and other rumors that from exs that he has had sex i really dont care if he had sex in the past but i just hate that he wont tell me the truth and how can i get the truth out of him?and i just dont understand why cant he just tell me!