Sex system in humans is part of their social system.
Like Swansont said, it is due to our evolution as h. sapiens that our mating seasons are not as pronounced as it is in other species. Greater apes also show some very interesting sexual behavior, and as I've mentioned it here once(forgot what thread that was in), but Bonobo's are much like humans when it comes to sex. That reason alone makes then a bit more diverged then some of the other apes, however strange as it is, it(their sexual system) alone does not make then more closely related to us.
Here are some more reads for those interested:
Bonobo Apes
Primate Males