I was listening to this seminar on marriage by Doug Wilson when he wanted to use Exodus 21:10-11. The video should start at the right point, but if not it's about an hour and 1 minutes into the video. He took 2 minutes to jump through all the normal hoops to get around polygamy until he came to the missionary scenario, where he says clearly the husband and wives can convert and not have to get divorced. The professor who told my class polygamy is not explicitly outlawed in the Bible personally knows and shares a lot of the same beliefs as Doug, so it makes sense they would come to the same basic conclusion.