Welcome to our forum.
I have no immediate advice; obviously you have to honor the wishes of Tina and Beth's husband. Just take time for now to learn as much about poly as you can. Here are some resources I recommend:
And here on Polyamory.com:
Opening Up covers the whole range of responsible non-monogamy -- including both swinging and poly, so it has a good point of perspective for your situation.
I think right now you just need to learn more about poly in general and see what you think about it. Then, you can share your thoughts and feelings with Tina, Beth, and her husband, see what they think, and what compromises might be possible, as needed.
On this site you can post your thoughts and questions as desired. It's a pretty busy site, so often you'll get multiple responses.
I hope your experience with us is beneficial.