Welcome to the forum, Mark.
To answer your question, if they are determined that monogamy is the only legitimate relationship form that isn't inherently sinful, then you are going to have a major problem making it seem "natural" to them.
The only thing you can do is to bring it up in casual conversation, not making a big deal out of it, but make a point of bringing it up soon during the evolution of a relationship. There have been quite a few threads on here about the right time to talk about it, and opinions differ.
"Listen, or your tongue will make you deaf." - Native American Proverb