There’s a strong possibility that the adult (adult, right?) daughter’s views on sexuality are very unlike those of her parents, or that they are in flux.
Here’s what I would suggest saying (if anything is going to be said):
Being a third wheel living with a couple, particularly in a relatively new relationship, isn’t a lot of fun for anybody. Even (or especially) if it were a legally married newlywed heterosexual couple, it would be weird and probably ill-advised to plan to live with them long-term.
You would be just as disapproving of her living with a heterosexual unmarried couple.
I don’t suggest saying a whole lot, because if the whole situation goes sour, you don’t want there to be such a huge pride thing involved that she won’t be willing to go to you and say, “My living situation is terrible and I hate being in their personal space all the time, when they’re fighting or when they’re lovey-dovey. I feel like an intruder in my own home. How can I get out of this?” You need to leave the door open for helping her later. It might be worthwhile to mention that you understand that roommate situations can go bad suddenly, and that if that happens, that you will be happy to help her find a better situation.
Huh? The fact that the environment is unhealthy is not dependent on her spirituality. She can still give her friends a good example without living with them.
DISCLAIMER: Catholic Answers has turned over the archive to Catholic-Questions.org and no longer owns, manages, or moderates the forums. For additional apologetics resources please visit www.catholic.com.