Marriage today is both a secular and a religious institution. Many atheists marry for the secular benefits. Plus it is a part of Western culture. Even though it may have its origins (in the west) in religion, it has become a part of mainstream culture. . . That aside, I have a lot of problems with your logic on this. If God created everything as you say, then how could atheists not be a part of something he created? Whether they married or not, they would be a part of something god created, right? If god created everything, as you say, he also created homosexuality, polygamy, and adultery and so people practicing those would just as much be a part of something god created.