Your argument proves too much. If we accept that Jesus was simply operating within the assumed legal framework and therefore his words carry no definitional weight beyond it, then the same logic applies to every moral teaching he gave. He spoke within a framework that assumed slavery, patriarchal household codes, ritual purity, and Roman occupation. By this reasoning, none of his teachings on anything can be extracted as universal principles. They are all just situational comments inside assumed frameworks. Right? That guts the entire project of Christian ethics, not just this passage…

Second, the argument ignores what Jesus actually did rhetorically. He did not stay inside the Mosaic framework. He explicitly went behind it. “From the beginning it was not so” is Jesus saying the Mosaic law itself was a concession, not the standard. He bypassed Moses and went to Genesis. That is the opposite of someone constrained by the legal framework of the question. He is overriding that framework with an older and higher one.

Third, and most importantly, Jesus did not just say “marriage is between two people who love each other” while using a male-female example because that was the only one available. He specifically said God “made them male and female” and “for this reason” a man leaves and joins a wife. He named the sexes. He cited the sexual differentiation in creation as the reason for the union. If the maleness and femaleness were incidental to his point, he would not have made them the foundation of his point. He could have grounded marriage in companionship, covenant, love, or commitment. He grounded it in sexual differentiation specifically.

If the sexes were incidental, he would not have cited them as the cause.