Confused... *UPDATED*

So my boyfriend and I got together 6 years ago and after the first time we had sex he admitted to me that I was his first. I wasn’t sure I believed it and while using his Facebook I discovered loads of messages suggesting he had done things with girls and slept with some girls but when I asked him about it he said it was his friends that wrote it, even though the messages had his name and he put his phone number on the messages. Why would he lie about being a virgin after we had slept today? The reason I do think it’s him is because I’ve seen him calling this one girl his girlfriend to his friend on a message and saying he was meeting up with her but yet he told me he’s never had a girlfriend before me. It’s just something that has constantly bugged me but he swears that it was his friends I just don’t understand..? I really don’t understand this but he refuses to talk about it, he gets angry saying I don’t believe him, but when there’s messages to suggest otherwise, how can I be sure. It’s so weird!

*update* We spoke about it last night, he swore on our children’s lives that it wasn’t him, his friends used to use his Facebook instead of theirs. I know he means it if he swears on something. He was too trusting with group of friends, gave out passwords etc. He thought they were like brothers to him, only to find out all they done was slag him off behind his back to each other so it makes sense that it was them