The Agni Purana speaks about inter caste marriages in details while talking about the Manus & the Varnashrama Dharma(the Dhrama of the 4 classes of society & the 4 stages of life).
Quoting from this page,
An anuloma marriage is a marriage where the husband is from a higher
class than the wife. The offspring of such a marriage belong to the
mother’s class. A pratiloma marriage is a marriage where the wife is
from a higher class than the husband. Chandalas were born this way
from brahmana women, Sutas from kshatriya women, Devalas from vaishya
women, Pukkashas from kshatriya women and Magadhas from vaishya women.
Chandalas are executioners, Sutas charioteers, Devalas guards,
Pukkashas hunters and Magadhas bards. Chandalas should live outside
the villages and should not touch those belonging to any other class.
So,naturally,a Pratiloma marriage is looked down upon and prohibited specially those which results in Chnadalas as offsprings.