While I respect other cultures and try to understand the different ways of life, I personally find the caste system a very inhumane way to unnecessarily segregate people and perpetuate biases that are harmful.
That said, you are living in it, not me, I can only give my perspective. If you really are in love, well then your family needs to learn to love you for who you are and what you want. If they do not respect that then their love is very limited and they will allow prejudices to limit their love for their own, which is very sad.
Now in American terms, you are both young and rushing into marriage is not necessary at this junction, but I do understand the cultural aspect and can appreciate the dilemma. My personal opinion would be if it is true love between the two of you, then you make your own family and life together. I know it is easier said than done in certain aspects, but it is not until enough people stand up for what is right that changes will start to occur. Look at the US, in the 60's "interracial" marriages were illegal in many states still. In the 70's and 80's it was still highly frowned upon. By the 90's it was becoming generally accepted, and by the start of the new millennium it has become common place and seen as a non-issue for most. So change is slow, but it does happen.
Again if it is true love, then consider leaving India and relocating together to start your new family.