In the Musaf of Rosh Chodesh, we mention that Hashem has given us "חוקי ראש חודש". What are these Chukim?
The laws of Kiddush HaChodesh, when to make 2 days or 1 day Rosh Chodesh, and when to make a leap year.
The Imrei Yehudah Al Hatorah (Tazria pg. 89) explains that we say:
This refers to the laws of family purity, which follow a monthly cycle. By G-d establishing the the laws of family purity, He has separated us from the nations. When G-d sees that we put aside our love for our wife because of our love for G-d, who commanded us to separate from our wives while they are in a state of impurity, He loves us.
So, we are saying in the Musaf Prayer that it is because of the laws of family purity that G-d chose us over all the other nations.
The Nitei Gavriel also says that the Darchei Moshe (417:1) says something similar, but I didn't check it out.