I really appreciate the amount of respect everyone here has shown.
After posting this and reading some feedback, I spoke with my husband about the idea of polyamory and we talked a lot. He has stated that while he adores her, he is not "in love" with her, and that's okay with me. I know that it may end up with "love" and...that's a bridge I will cross when we get there, but for now, I'm doing a lot of thinking about it, and what it might mean to me.
I will be meeting the girlfriend and her husband for the first time tonight. We're planning dinner and a movie and maybe some board games.
I want this to work - I want my husband to be as happy as he can be; I don't want our marriage to be about limitations, but rather how high we can fly with the support of one another.
Thank you again, everyone.