This is an interesting passage from the first-century BC Greek writer Diodorus Siculus, both for the ideas he is promoting in regard to India and for the assumptions he brings to a discussion of marriage. On the practice of burning widows, see further Wikipedia.
It is an ancient custom among the Indians that the men who marry and the maidens (παρθένοι) who are married do not do so as a result of the decision of their parents but by mutual persuasion. Formerly, since the wooing was done by persons who were too young, it often happened that, the choice turning out badly, both would quickly regret their act, and that many wives were first seduced, then through wantonness gave their love to other men, and finally, not being able without disgrace to leave the mates whom they had first selected, would kill their husbands by poison. The country, indeed, furnished no few means for this, since it produced many and varied deadly poisons, some of which when merely spread upon the food or the wine cups cause death. But when this evil became fashionable and many were murdered in this way, the Indians, although they punished those guilty of the crime, since they were not able to deter the others from wrongdoing, established a law that wives, except such as were pregnant or had children, should be cremated along with their deceased husbands, and that one who was not willing to obey this law should not only be a widow for life but also be entirely debarred from sacrifices and other religious observances as unclean. When these laws had been established, the lawlessness of the women changed into the opposite, for as each one because of the great loss of caste willingly met death, they not only cared for the safety of their husbands as if it were their own, but they even vied with each other as for a very great honour.Diodorus Siculus 19.33 (LCL)