I see some validity to this - not from a "high/low" perspective - but due to this selective non-inter-caste breeding over the centuries, there are some traits specific to certain castes...

Also, there's the whole families/relatives pressure and stuff. Plus, with faith tradition differences - say Muslim/Christian/Hindu, there can be challenges.

Maybe I am wrong but as open-minded as I am, I generally don't think I'd marry a Muslim or a Christian - mostly for the familial challenges it can bring. The SC/ST side of things, I'm not sure yet. But again, all of this could change depending on who the person is and what values/principles they hold. I, myself, am areligious after all - while I'm 'supposed to be Hindu', much of my spiritual saadhana is based on Buddhist ideas, Christian prayers, and Hindu/Islamic asceticism also.

I think the marriage part when it comes to castes is far more nuanced and complex - to-marry(+kids)-or-not and who-to-marry are arguably life's 2 most important decisions, so a lot has to be taken into account.