Whatever you see or not, marriage as an institution evolved in societies independently, worldwide for a reason - societal stability. It goes in parallel with evolution itself, is a subset thereof (cultural evolution) and needs no appeal to superstition / deity to see its validity and justification.
I see. So sin is that which is seen as promoting societal stability for the preservation of the species. It certainly does, and it is not as if God had no reason for requiring that sexual union be between opposite genders in a life long covenant, thus providing the security that sexual vulnerability and intimacy should have, and for the normal result of that.
But atheists can just as well argue that fornication is not immoral, and in fact at least one poll finds the substantial majority affirmed pornography and cohabitation as a moral behavior. And thus it is no surprise to find that the marriage rate is much lower among atheists.
Moreover, they can just as well argue that incarcerating or otherwise removing children from the homes of evangelical Christians, or even a certain race, would be best for societal stability and preservation and advancement of the species.
Certain one can do the same upon a religious basis, invoking their respective "holy book," yet all religions are not the same, while at least there is an objective supreme transcendent standard to interpret.
In so doing with the Bible, one can hardly justify flying airplanes into the WTC, or the Inquisitions, which actually was executed under the premise that the church (like as in cults), not Scripture, was supreme, and which hindered literacy in it by the laity. And which literacy would have its outworking in forsaking the use of the sword of men by the church to deal with theological nonconformity. which early Protestantism had to unlearn.
However, in atheism there is no objective transcendent supreme standard to judge by, and what seems reasonable to an atheist can be that of a Mao Zedong or a Mussolini, etc., under the premise of what is ultimately best for societal stability and advancement of it.
This does not mean an atheist cannot be a relatively moral person, even more so than a religious, but this is judged upon a basis for morality, which cannot simply be what seems reasonable to each individual, but based upon a standard shown to be so when obeyed.