Even if it wasn't openly discussed, it's still obvious. After they had lived in the Underworld for 200 years, in the arch of Alicization, and they hadn't quarreled, broken up, or killed each other. 200 years old! Kirito is clearly good at sex, communication, and how to make Asuna happy. Relationships have passed the test of time. I think if other girls besides Asuna had fallen into this trap inside the virtual world, they would have persuaded Asuna to create a harem. 200 years is enough for Asuna to give up.