For the faithful Catholic, this is simple: one man and one woman in a lifelong bond. We initially take that from Scripture, when Jesus is explaining to the Jews that there is no marriage in heaven and that they only had divorce because of their hardness of heart. Explaining to them that it was never meant to be this way, he retold the story of Adam and Eve and reaffirmed that their bond is how marriage is supposed to be; therefore, Jesus Christ did redefine marriage - he changed it from a social construct into a Sacrament. Modern man, however, has reversed it, changing it from a Sacrament back into a social construct.
The atheist can ask: But still, which version of "Biblical marriage" is the correct one? This is where the Catholic Church has the advantage over the non-Catholic world: we have the Magisterium, the infallible teaching authority of the Church. Whereas the Protestant world uses the Bible as the final authority (the theology of sola Scriptura), the Church follows St. Paul's letter to St. Timothy, where he calls the Church "the pillar and foundation of truth." St. Paul, therefore, explains that it is the CHURCH who upholds and defends truth, allowing the Church to have the final say - and the Church says marriage is where "a man and a woman establish between themselves a partnership of [their] whole of life", and that "by its nature [it's] ordered toward the good of the spouses and the procreation and education of offspring; this covenant between baptized persons has been raised by Christ the Lord to the dignity of a sacrament" (CCC, 1601).
Here's where non-Catholics and atheists can agree: "Who gave you the authority to declare such things?" For the faithful Catholic, we know who: Jesus Christ. "Christ is himself the source of ministry in the Church. He instituted the Church. He gave her authority and mission, orientation and goal..." (CCC, 874).
So, if you take authority to interpret Scripture away from the Church, which was divinely created by Jesus Christ, and put it in the hands of just anyone, then you can have Christians defining marriage one way and atheists defining marriage another way. For the faithful Catholic, it's very simple: marriage is a Sacrament because Jesus Christ said so, and His Church has continued to define and defend it in the very same way as her Founder.
It's all very simple: Marriage is a life-long bond, a Sacrament, between one man and one woman. Why? Because God said so. How do we know? Because the Church He founded teaches that. End of story. End of debate. End of silly billboards and offensive memes. Case closed.
"...the Catholic Church professes that this faith, which is the beginning of human salvation, is a supernatural virtue by which we, with the aid and inspiration of the grace of God, believe that the things revealed by Him are true, not because the intrinsic truth of the revealed things has been perceived by the natural light of reason, but because of the authority of God Himself who reveals them, who can neither deceive nor be deceived" (DF, 3).