Is it normal or an excuse to cheat?

I have been married to my husband 4 years. Prior to us getting involved with each other he had one long time girlfriend of whom they had a really dull sex life. When we got together,our sex life was amazing. He was amazed on how fun sex actually could be. He has become pretty sexual since we started our relationship,and often refers to himself as the 26 year old virgin. (that's how old he was when we met) about a year ago, he started asking about my sexual history and wanting to hear stories about it. Then he confessed to me that the idea of me having sex with a another man while he watched turned him on. I love my husband and have always wanted to help him with his sexually exploration and fantasies. I agreed to try it. The first time,i actually cried after and felt really bad. He apologized and said he would never bring it up again. He did not touch the man,he just watched him have his way with me. The man left & then we had amazing sex. I just felt really dirty after and ashamed. Since then,he had brought it up a few more times and we ended up having a MFM 3 some about 3 times now. We really talked about before we did it again & i explained to him my feelings of worry. Because the idea of him having sex with another women,would really hurt him. He always say,i love you & i have no desire for another women. I just really love to see you "enjoy" yourself. I love you. I tested that fact out & actually tried to have a FFM 3 some. Me and the girl played around & he just watched. He did get aroused but refused to touch the women. It did make me feel better. But in the back of my mind,i keep saying this is so wrong. I do get sexual pleasure out of seeing him get pleasure from watching. its just after i always feel a bit of shame. If anyone knew of what we do,i would probably die! i grew up in a really religious family,so maybe that has a bit to do with the shame. My question is,am i crazy for always thinking he is using this an excuse to one day ask to screw another women? Or are men really into MFM 3somes purely for the idea of the women getting sexual gratification from a larger more endowed man? Is this normal? I'm just really confused about it all,i did it mostly to please him. & i get pleasure from that aspect. (of course the sex isn't to shabby either)