human beings are also animals and do you see any other animal spending their entire life with just another animal ?!
Human beings are classified as homo sapiens and that means higher animals. Our senses and every part of our body is far developed and more prepared to handle , control, dominate and subdue our environment. Our precision is far above even the orangutan or gorillas who are considered closest "cousins" to homo sapiens. Our precision and cranial capacity are why we have our environment at our fit and using it to better our living on earth producing better technology to fly on the sky and below water bodies
So with the marriage thing, it started from creation, the bible when God saw Adam was lonely, he made a woman from him and for him and they started living and having sex especially in confines when they realized it was made to be "HOLY ACT"
. That brings a question here; how many animals you see having sex in a hidden place or a closed shelter? They do it anywhere which is why they can mate with different others. Human beings are not meant to have sex in public in a moral sense except for now we see things changing and porno films and clubs are everywhere in our eyes.
Hence, the society has made marriage to be a more defined setting for sexual gratification done in confines and couple means people who are identified as enjoying certain level of privacy and gratification alone. In Africa and Nigeria, marriage is a serious affair, a union between man and the woman, although remember there are different types of marriage in Africa also.