<human>: Give me the gist of the text in a nutshell

So basically, my boyfriend and I have been together for 2 years and 3 months and as far as I knew he was completely happy and satisfied with the relationship.

Last night, he left his email account open on my laptop and as I went to log out, I noticed that some of his most recent received emails were from craiglist ad replies with subjects like 'curious couple'. I opened them and saw he'd posted a m4wm ad looking for couples who wanted threesomes. He said he was bicurious and open to doing stuff with men, which is something he's never told me. In response to one of the replies asking for photos, he'd sent nudes.

I confronted him last night and he is denying *everything*. He says he doesn't know how the emails got there (when I saw them, they had already been read) and says it's not him replying to the messages. The email signature say they were sent from a Samsung phone, which is what he has and emails from most of the time.

He definitely has the type of friends to do this for a joke, but the thing that has put me off is the fact that nude photos have been sent that I really don't think his friends would have access to.

We have sex regularly and we've tried a lot of things both of us have never done before. We're not private about discussing fantasies and he knows I'm open-minded about sex and sexuality. If he is bisexual/curious, it wouldn't be an issue for me - the thing I'm upset about is him seeking out sex outside of the relationship and not telling me the truth. I don't understand why he won't admit it, and it's really upsetting me. We had/have plans to get engaged, I'm completely in love with him and couldn't imagine breaking up. I want to work through this but he's being so difficult by not admitting he's at fault.
<bot>: my boyfriend posted a m4wm casual encounters ad on craigslist and all evidence points to it definitely being him posting it, yet he continues to deny it. Wtf do I do?