Marriage may be a sacrament to catholics, but it existed in one form or another before christianity, and it exists in different religions and outside of religion (remember, you can be married by a justice of the peace, and if i remember correctly, most religions will now view that as the same sort of bond). Just because CATHOLIC marriages are 'supposed' to be between a man and wife, doesn't give anyone the right to say what they can or can't do in a civil marriage.
I figure, if they're not doing you any harm, what the hell is the big deal? They're not hurting anyone, just trying to be happy....... just like everybody else. Let them get married and live their lives in peace - it's not like common law isn't virtually the same anyway. It's just a piece of paper, if they want one, let them get it and keep your noses in your own business.
My 2 cents