Sex creates bonds. It gives us reasons protect one another, to bond together as a group to work together for common goals. It is part of what creates society.
Reproduction is a part of that, but the whole is so much larger.
I'm wondering why you chose the bond forming part of sex as the primary purpose, as opposed to reproduction. Also, the bonds created secondary to sex are supposed to promote monogamy and I'm wondering how that automatically leads to a society working together for common goals.