Children, new partners and nudity

I have a good relationship with my 5 year old's father and his new girlfriend. They've been together for about 8 months and have just moved in to a new house. He does, however, seem rather keen to think of the new girlfriend as 'family'. My daughter loves spending time with them, but recently returned home with tales of playing with the new girlfriend's breasts in bed in the morning, and of all three of them bathing together. I know that concealing nudity isn't always a good thing, but I can't help worrying about this a little. While there's definitely no abuse going on, it still seems strange. How do you think I should approach this?
Many thanks!

You can ask them in a friendly way to listen to your arguments. You can also ask your daughter not to accept nudity when she is with her father.