Your situation is uncannily similar to ours!
My husband and I have some friends who just recently came into opening their relationship. They had interests in the two of us but we respectfully declined saying that we thought we would better serve them by being friends they could talk to about their adventures, rather than be apart of their adventures. The flirting continued which was not a good idea I might add as it confused the issue.
So they eventually found another couple on line that was 15 years younger than them. They are in there mid 30's. They wrote an excited email the next day about it which precipitated in a long conversation.
Its important to remember not to do things for others because it is expected. There is always an out and its better to feel like your integrity and self worth is intact than to feel used, cheap and slutty. My husband and I have had our time as swingers and in an open relationship and have come to know some of the lasting negative effects. It was exciting to feel desirable and sought after for sure, but I ended up feeling used. Please tell your friend that casual sex with people could increase her issues rather than solve them. She won't get the loving attention she deserves from swinging and casual sex. She can get that from you as her friend more!
Stand firm about what you need to see happen because you will be of better use as their friends right now. You can certainly use the baby as an excuse! You won't feel all that into it when it comes anyway! Trust me!
(assuming its you having the baby?)