Should I trust my gut or him? Is he lying and cheating?

I've been dating this guy for a while now and he's great! We hang out a lot, he's sensitive, romantic, charismatic, and treats me well, but there's just one problem…
We were hanging out with a few friends one night and I went into my room to check my phone… We both have white iPhones, and so right when I walked in my room, a phone went off, I automatically assumed that it was mine… Well, it wasn't. I opened it and by the time I figured out that it wasn't mine, a message popped up from someone. It was another woman. Not only have they been texting all day, they were sexting all day. I broke down. I've always been really hesitant to trust someone and the only time I do, I end up getting hurt. My friend walked in and I told her what had happened and then my boyfriend walked in. My friend left us to talk. I told him what happened and he swore that it wasn't him. He claimed that his cousin borrowed his phone to text this girl. Which doesn't add up because he was texting ME all day with out a hesitation or a break. Anyways, he went on to say how he wouldn't ever do that to me and blah blah blah. So I trusted him and moved on… Well, a week later his phone was on the table as were talking and it buzzed he picked it up and moved it away from my vision really fast but slow enough for me to glance at the name and the basics of the text message… It was from the same girl and she sent him a picture.
I started to feel sick to my stomach so I nonchalantly started up a conversation about relationships and cheating and asked what his opinions were. He feels really strongly and sensitive about it and claims it to be a miserable, terrible thing. I don't know what to believe… My gut or him?

Trust your instints, dump him

I didn't read to the very end…
but always stick with your gut…
don't accuse him… Just tell him you feel uneasy
if you have a premonition it's for a reason and you NEED to listen to it <3
answer mine please.

Leave him, he is a player and you can sense it, already!
Leave him while you are not too attached to him. He can ruin your life! Do not trust him. You saw the sexting what else do you want to stay for? To be his last resort.
You deserve better. Trust me. Walk out on him. If he doesn't want to be exclusive…
He's had his chance with you and he blew it.
Those guys always find a reason to keep you…
But not to make yourself feel good but it's for their pleasure mainly… These are the ones ruining other people's lives, in the end. They say they never promised anything, when being told that they said they would be there for you.
A miserable, terrible thing? Maybe he made that girl feel miserable and you will be the next…
Trust your instincts, most of the time, they are right…