The whole idea of being mono is that you only want one person to have that intimacy with. Why he isn't comfortable that you are poly is probably because most people who are brought up around all this mono culture, feel that it is even wrong to be with more than one person. But it is something people can overcome.
I know it's a lot more complex than that really, but it gives you a basis. I think it could be an interesting question to ask my partner. =]
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