My boyfriend is bisexual?! HELP?

I have been dating my boyfriend of four years. I always new he was a little metro and open minded because of things he done but i never really thought twice about it as he is the only guy i have ever really been with. I was told recently by a friend that they heard he was chatting to some girl about cheating on me. I instantly freaked and grabbed his phone. I found a message between him and some girl about him saying he had been thinking about cheating on me alot lately and "aren't you supposed to be bringing me to the sex shop soon". I instantly left not know what to do or think. I contacted this girl hoping to hear I was over re-acting. She told me that he talks to her quite frequently because he is comfortable with her. He is Bisexual and she has told him many times to tell me. She didn't go into detail. I had him come meet me and he said it was all fake and just a fantasy that never happend or actually would. He says he loves me and only me and wants to be with me forever and will never think that way again because he wants a normal life with me. He said he is scared i will tell people. We are about to move in together. He cant afford the mortgage by himself. i know i love him alot an he loves me but now i keep thinking if he really is in love with me and wants to be together forever or if hes just scared i will tell someone or not move in leaving him with the mortgage payments. I told him i think he needs to figure it out because i do not want to be with him if later down the road he is thinking about "experimenting" because im just not ok with it. he said he loves me and only me. I am really scared to move forward knowing what might happen but i dont want to just throw away what we have without knowing and understanding this.

any insight will help!! i am extremely confused. I do think he is by but not to the extreme. i want us both to be happy wether its together or not. I am so on the fence with this it really came out of left field for me and i have nothing against gays or bi's but to date one is a completely different feeling. we broke up last year for a few monthes but since then i thought we were doing amazing and why would you get back togehter with someone after three years if you were unsure. and then this all pops up and he was messaging her back two monthes ago. we don't even live together yet ... he bawled his eyes out yesterday and i have never seen him like that let alone cry. i do believe he really wants it to work but i dont think you can hold that in forever. i dont want to be ignorant either....

HELP!!