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 184.108.40.206-5 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.
Varnasankaras are product of intercaster marriages. In the first chapter of the Gita, Arjun described that such varnashankars destroy the 'kukadharma' and take the members of the whole'kula' to hell. Manu says: one reason of varnasankara is a marrige between a lower caste boy and a higher caste girl. According to the Naradasamhita, also such pratiloma marriage creates varnasankaras. Note that Aryuna mentions kuladharma as saswata ie eternal ie something which must be preserved.
Manu accepts anuloma marriage (स्त्रीरत्नं दुष्कुलादपि).Tantrasaara says that if a shudra male marries a brahmin girl, he becomes a chandala.