Answer: Boss-employee romances always create problems. Talking directly with the participants is pointless, since people in the throes of lust are seldom rational. And when one of them is your boss, there's a risk of retribution.
A better option is to find someone in upper or human resources who can sit this new manager down for a frank talk about inappropriate workplace relationships. When you meet with that person, keep the tone calm and businesslike.
For example: "Our group used to be very effective, but lately things have changed. There’s a rumor that Bob and Mary are having a relationship outside of work. We don't know if that's true, but the two of them are often gone for hours, and Mary has become difficult to work with. We're afraid to discuss this with Bob, so we hope that you can help.”
Although your cautious colleagues may try to make you the sole spokesperson, they should also attend this meeting. When several people deliver the same message, management is much more likely to pay attention.