Anuloma marriage means higher Varna men marrying lower Varna women. Pratiloma marriage means lower Varna men marrying higher Varna women. What is the reason behind encouraging Anuloma and discouraging pratiloma marriage?
We have Varna system. As per this, Apastamba Dharmasutra 220.127.116.11-5 says:
[There are] four castes Brahmanas, Kshatriyas, Vaishya, and Shudra. Among these, each preceding (caste) is superior by birth to the one following.
So, it says:
For a Shudra, preceding varna Brahmana, Kshatriya and Vaishya are superior.
For a Vaishya, Preceding varna Kshatriya and Brahmana are superior.
- For a Kshatriya, Preceding varna Brahmana is superior.
And among husband and wife, husband has the superior authority. But if Pratiloma marriage and takes place, we don't know who is superior(The girl is because she belongs to higher Varna or the boy is because he is the husband). So Anuloma marriage is recommended and Pratiloma marriage is discouraged.