I think that ultimately people only come to see this if the Holy Spirit prompts them to. Without His prompting, they'll stick with their emotional preconditioning and read monogamy-only into scripture, and will never be dissuaded from that through logic because emotions are stronger. And for most people it isn't a crucial matter, there are more important aspects of Christianity than this. So I don't try to persuade everyone, just offer pointers in the right direction for those who are of a receptive mindset already.
For those, the most effective way to shine light on this is to ask them to prove from scripture that only monogamy is acceptable to God. If God is leading them to think deep enough about this, they'll honestly find that they cannot prove it. If they instead grab some proof-text that they think proves monogamy-only and stick with it even if you show their logic is completely flawed, then they're not ready and you may as well drop the subject. Unless, of course, polygamy is actually relevant to their life in a real way and they do need to be persuaded for practical reasons, in which case you'd persist with trying to make them see sense.
For the potentially receptive, you can help their thinking by pointing them to some thought-provoking texts like 2 Chronicles 24:2-3, and by pointing out that in the original language verses like 1 Co 7:2 don't say what they appear to say in English.
but open to what ever you think should be shared!