All without actually caring, I'd remind myself (and them, of course) that they'd been adopted. And then I'd probably get to work on a good old familial guilt trip on the poor sod and/or hen that would hopefully result in a new bike in my garage and my holidays sorted for the year.
If you mean multiple casual partners as in they were cheating, I'd call whichever partner they were with that day by the others name.
If you mean they're basically just sleeping around with no commitments, I wouldn't be particularly thrilled but most people have done it at some point. As long as they're safe with it and such, it's their life.