In Bhagvata Purana, Canto 4 the status quo and genealogy of Manu's daughters is given. You could read it online here.
Apart from his sons Priyavrata and Uttanpada, Manu had three daughters from his wife Shataroopa. These were named Akuti, Devahuti and Prasuti. Great sage Ruchi was chosen as the marrage partner for Manu's daughter on the condition of 'Putrika Dharma.'
Putrika Dharma was a condition on which only a father with no sons could insist at the time of his daughter's marriage. In this custom, a father without any sons could take eldest newborn child of his daughter after her marriage to help his family.
Akuti and sage Ruchi's son Yajna(sacrifice) was incarnation of lord Vishnu. Manu took Yajna away to upbring him under his care. Their(Ahuti and Ruchi's) daughter Dakshina(charity) was incarnation of goddess Laxmi. Due to the obvious attraction between these two they were married with each other.
This is an example of marriage between co-borns in Bhagvad Purana. Many such examples exist in Hindu scriptures to prove that such customs existed in previous yugas. Later on, perhaps because of the need to get diversity of genes, inter-family-caste marriages started getting much favor and ultimately became Dharma. You are ostracized from standard Bramhin communities if you marry an outcaste.