Help getting him to come out!

Hi everyone,

I am a 31 year old, openly bisexual woman, in a long-term relationship with a man for the past 3.5 years.  He knows I am bisexual in the past, our relationship has been open, and he has encouraged me to date women.  During the course of my relationship with him, I have had 2 girlfriends, dated others, and all the while, he has been supportive of me.

Until yesterday, I thought our relationship was pretty honest; we are open with one another about everything, including our sexual desires.

I realized last night that this is NOT the case; he is away for business this week and we decided to go onto a chat.  This didn't work out for us because his chat program was not working on his laptop.  However, when I logged on, it was his screen name that was signed in on my computer (we do not live together, but he is often here).  When the welcome screen popped up, I saw names of many people, both men AND women with whom he has chatted.  I admit it bothers me a little that he has cyber sex with other women, but that is another discussion for another time and I am not a hypocrite (if I am having sex with women, he can, too). 

Unfortunately, I let my curiosity get the best of me and I started snooping a little.  I discovered that not only has he been chatting and sharing explicit pics with women, but also men.  LOTS of men.

I am not upset by the idea that he might be bi, I have actually suspected it from time to time, and tried to ask leading questions that might make him admit it, but he is always adamant that he is 100% straight.  What I am upset about is the fact that he is hiding this from me.  I am totally open about my sexuality, and try to make him feel as comfortable as I do.  Why can't he talk to me about this?  And of course, if he can't even tell me he is TALKING to men, what else might he be doing?  I am ok with an open relationship, but it has to be full disclosure.  I am extremely uncomfortable with the thought of "what could be."

Any objective thoughts or advice about this?  My head is spinning and I am afraid my imagination is going to get the best of me.

Thanks,
K