Gita 1:40 (Arjun says)
From the predominance of sin , O Krishna, women become corrupt , and then O descendant of Vrishni, cross-breeds are born.
Above verse of Gita indicates that when women become corrupt , they produce Varna-Sankaras.
No where it is written that Varna-Sankara is produced when they marry people of other caste. It is just written that Varna-Sankaras are born when women become corrupt.
Manu Smriti 8.352. Men who commit adultery with the wives of others, the king shall cause to be marked by punishments which cause terror, and afterwards banish.
8.353. For by (adultery) is caused a mixture of the castes (varna) among men; thence (follows) sin, which cuts up even the roots and causes the destruction of everything.
From adultery and sexual intercourse between prohibited castes arise Varnasankaras.
What the verse you quoted means is, women becoming "corrupt", being over driven by lust, are prone to commit adultery and which will result in Varnasankara progeny.
It does not mean that the child of a Brahmin man and a Brahmin woman will be a Varnasankara if the Brahmin woman is "corrupt". Varnasankaras are only created out of intercourse (adultery) involving prohibited castes.
Yes Corrupt women produces Varnashankars , even if both man and woman belong to the same caste.
If the wife was non-virgin before being wedded to the husband or she had "belonged to some other man" , the child she will give birth to will be Varanshankar.
Manusmriti 10.5: Among all castes, those only who are born of consorts wedded in the natural order, as virgins of equal status, are to be regarded as the same (as their father).
Āpastamba (2.13.1).—‘Sons begotten by a man who approaches in the proper season a woman of equal caste, who has not belonged to another man, and who has been married legally, have a right to follow the occupations of their castes.’
And a Varnashankar child is useless. As a father can't be said to be reborn if the child is Varnshankar. As he can't follow the occupation of the father.
Gita 1:40 (Arjun says)
From the predominance of sin , O Krishna, women become corrupt , and then O descendant of Vrishni, cross-breeds are born.
Gita 1:41 (Arjun says)
Such cross breeding leads both the exterminitor of the race and the race itself to hell. The deceased ancestors whose sradha ceremonies have ceased (to whom children have stopped to give food and water) fall (from heaven to hell)
