Is it normal that my bff, claiming to be completely straight, acts so gay?

Hi, I am a bi guy (but prefer guys recently) and I have a best friends. We are friends since summer. He claims to be completely straight, he loves sex with girls, he has a girl friend, and she is also a good friend of mine. During the summer, we used to hang out almost every day, he used to stay over, we were drinking and smoking a lot. We had fun. Then he started to flirt with me...hugs, punching my ass a little, cuddling in bed. Once he kissed me on the cheek because I was caressing his stomach and he thought I was his girlfriend. His girlfriend was jealous of me, they were fighting because of me, and so on. Then I left for college in another state (more than a 1000 miles away). He used to say how much he missed me. I got home for fall vocation, we spend all the time together. He and his gf were fighting again.....all because of me. I left for college again.
Then we lost contact for a while. I was busy because of school, he worked a lot. I got home again for Christmas break, first week we barely talked, we were fighting a lot. But things got better, we used to hang out a lot. But this time his girlfriend didn't let him stay over at my house. Once, I had a big party at my place. They both stay over, he asked me to sleep in a same bed with them. First he was all over his girlfriend, but when she felt asleep he started to cuddling with me. We were holding hands all night. He even tried to put his hand inside my boxers, I stopped him. He did it again that night, but I stopped him again. He said sorry and we never talked about it anymore. Maybe he though I was his girlfriend. But anyway, we do things that straight guys wouldn't normally do- sleeping in the same bed (big bed, but we are always close to each other), holding hands all the time we are alone, he kissed me on the cheek several times, he bites me (ear, nose, neck), he plays with my nipples but I can't play with his, he calls me "hun" or "teddy bear". He says he loves me (in a friend way) all the time. His girlfriend says we act like a married couple. I know he loves her, but that's somethings weird about this. Many times his girlfriend went out, but he rather stayed with me. He asks me to caress him all the time. Once when I gave him a massage his cock got hard. Once he was telling his girlfriend if she doesn't have sex with him, we will do it with me. And again, his cock got hard. When we sleep in the same bed, I always put my hands inside his underwear, his cock is almost always hard. A few times during the night he was acting out sexual movements at me. We talked about this, he even told his GF. We all were joking about this, his GF said he did it to her all the time during the night. He also wear my clothes (even underwear). When I was leaving I gave him one shirt, he said he will never wash it because he wants to smell to it. When we were skyping last night, he kissed web cam and got mad because I din't want to kiss my web cam. Btw, I am 19, he is 21. He used to be homophobic, once (many months ago) I told him I liked him (in a friend way). He said it was gay. But he has change a lot since that. He acts completely different when he is with me...and when he is with his guy friends. I know he seems to be BI, but when someone says he acts gay he always reject it. He also doesn't know I am BI. I think I'm falling in love with him. But I don't wanna tell him. I know this might ruin our friendship, and it would also ruin my friendship with his girlfriend. But I don't know what to do. Any ideas?