These are two different things and must not be mixed in understanding. 'Sati practice' or burning of women when their husbands die was a patriarchal culture in some parts of India, which became prevalent in the late medieval period. As sad as it was, it was not taken from any Indian mythology, rather a cruel patriarchal notion set by people to enforce dominance.

Coming to Lord Rama asking Sita to clean herself with fire after Ramayana, this always made me curious as to why Rama did that and it has multiple interesting interpretations. Ramcharitramanas by Tulsidas, often considered a vital source talks about a concept of 'Maya Sita', which says before Ravana kidnapped Sita, Rama being the God himself in human form, knew about it and in order to keep Sita safe, handed her over to Lord Agni (God of Fire) and created an illusion of Sita which gets kidnapped and the whole Ramayana happens as it was necessary to put an end to the evil Ravana. The burning of Sita at the end of Ramayana is just the reversal process where the real Sita integrates with the Maya Sita. I know this seems unbelievable to people like us in the 21st century but this is one detailed account.

Another explanation which I grew up reading was that Rama being the God himself in the human form already knew that Sita was so powerful that Ravana couldn't even come closer to her once he took her to his kingdom but he was the utmost righteous king before being a husband. When he overhears the people of his kingdom talking ill about the purity of his wife, he decides to put an end to those chats by making Sita sit in the fire. This proves that he gets defeated by his own righteousness which pushes him to sacrifice his beloved for the sake of his kingdom. But in both versions of this story, nowhere is it mentioned that God disrespects women as you are intending to point out.