How do I know if my relationship is ending!?!

So here's the deal...I am a bi-sexual guy and so is my boyfriend. I know that he wants to sleep with women and he hasn't (that I know of) because he knows I wouldn't like it. It seems like our relationship has been going down hill for the past couple months...we only have sex like maybe a couple times a month...he says its because he's just not always in the mood for sex...I feel like he doesn't feel like sex with me...I did something that I'm not proud of last night and looked through his phone. I found a few things that he never mentioned to me...one was a text conversation between him and a female that he has hooked up with in the past...she asked him if he is in a relationship and he told her no. Then she asked if he was interested in casual sex with her and he said yes. Does that mean he has full intention of being with her and hiding it from me? I have always told him that he can sleep with women if he needs to, but he always said he would never hide it from me...We have talked about finding a women that would be interested in being with both of us at the same time...Its not something that is easy to come across but we haven't tried enough to say we really gave it a chance! We are together most of the time so I know that he's not out sleeping around all the time, but its hard to believe that he only wants sex once a month! He also has a profile on a bunch of gay and straight dating/hook up sites...It doesn't look like he is very active on them, but why is such a secret!? I don't know what to do. I love him so much and i do believe he loves me, but i just am not sure if i can completely trust him or not. What do you think? Any suggestions are appreciated! Thanks!

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