In this shloka from Srimad Bhagavad Gita (Arjuna Vishada Yoga/Chapter 1/Shlokas 40,41), Arjuna is worried about the consequences of the war. With widespread death and destruction that the war will wreck, Arjuna worries that several kulas will be destroyed, and with them, the kuladharmas, that have been followed for a very long time. With the destruction of these dharmas, he fears that widespread adharma will follow. With widespread adharma, women will become corrupted, and subsequently, intermingling of the various kulas/varnas will occur.
These shlokas emphasises the importance of kuladharmas and the extent to which varnashrama dharma was followed and respected in BharataVarsham — one of the first concerns Arjuna had, as he embarked on the war!