My (22F) dad keeps calling my boyfriend (23M) my pen pal?

I’ve been in an LDR for about a year now, we were friends at uni for about two years before we got together. My dad calls him my pen pal, and I think he means it to diminish my relationship. I can’t tell my dad this upsets me because he doesn’t take me or my relationships seriously, which I know means I shouldn’t care what he says. But I do.

Does anyone have any advice about feeling less down about this?