marriage

I personally am not religious... at all. Marriage is a piece of paper for the government in my eyes. (For those who feel differently I respect it. I just don't feel the same). I have a long term relationship, with someone I can not live with out! That's enough for me. We are expecting our first and we're both over the moon. My SO's dad is religious and very upset because we aren't married and "what will people think" no joke. Has anyone been through this? How'd you get the other person on board with out disrespecting how they felt?