Why is my relationship with my 21 year old girlfriend such a big deal to other people?

I’m 32 and my girlfriend is 21. We get along well and we are of both legal age. The thing is so many guys and girls say to us that we shouldn’t be together

What is the big deal? Why does this happen? Like me and her being together doesn’t impact anyone directly except us so why does it matter?

Why is my relationship with my 21 year old girlfriend such a big deal to other people?
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