The plot runs as follows:
It starts at a wedding where a man (maybe 40 or 50) meets a woman (similar aged) and they flirt a bit while smoking and it slowly becomes clear they know each other. The wedding slowly ends and both want to go to their rooms but end up going to her room and spend the night together. It turns out she has a family in Europe and he is also in a relationship with a younger girl.
The movie is a bit older and most things happen within the few rooms of the hotel.
Sounds a bit like I’s Complicated 2009 with Meryl Streep