Friendship

Can you make new friends of the opposite sex in a relationship?

My boyfriend stated in the beginning of our relationship that there was no need to make new friends of the opposite sex 2 years later he now has a female that is single and they were friends until he told me and I told him no. Now he explains he didn't think it was possible to have a platonic friendship while in a relationship until now. I don't know if I'm over thinking but I don't like it.