Great question- Why does Jesus say that women can instigate divorce? But the passage has a lot of complexity. The standards for men and women are different. As David Parker writes in The Living Text of the Gospels (1997): “For a woman it is remarriage; for a man it is the prior step of divorce."

I'm not sure I'm getting your suggestion that Roman law is being referenced. It seems out of character for Jesus. What would be in character is for him to speak as a Jewish prophet.

Christians often don't understand that "marriage" and "divorce" were how Jewish prophets discussed the relations between gods and nations. A deity is called a 'baal', which is just the regular word for a husband.

There may be a context here which is not marital law, but rather, the messiah dealing with problems between gods & nations. The nations (humans) must 'divorce' their gods and go over to Jesus the new king.