I don't want to impose the value on people that don't share it with me. I want to convince people to share the value with me first and rely on network effects to convince others to share the value until those that don't share it are in a minority.

Yes but how? Is it really realistic to expect the people in this story are going to be talked into monogamy?

The people specifically in the story? Probably not. Americans in general over a period of time? Maybe. I could see a "religious renaissance" happening at some point where religion goes on the rise again if/when people become disillusioned with secularism/nihilism/etc, a side effect of which would be people being "talked into monogamy"

The actual trend with religion is precisely the opposite.