Let me rephrase...
No, I do not think it is acceptable to date multiple partners.
In reference to "You are probably in favour of polygyny as well" is an assumption.
Unacceptable by whom? Society? Or the willing partners who were informed of it before signing on?
Personally I think it's a dumb idea as people's emotions are and always will be involved. But there are tons of dumb ideas that are perfectly acceptable in life. I just don't like the fact that people want him to force to change. Be honest, yes. Change? That's not cool.