Sex is meant to be a good thing. Barring the attitude that has been held about sex throughout many ages, it's not the root of all evil.
Sex was in the Design, you understand. God meant for us to have sex. Not that that means we should be running around boinking everything that'll stand for it, of course. But sex was a part of The Plan. Right? And, since we operate under the assumption that God = Good, and everything that God created was declared good, by God, then the functions of the things He created must, perforce, in their natural places and usage, also be good. I get that the assumption that God = Good is a rather large one, but it's the one we have to work with. After all, if we assume the opposite, then our moral laws are given by an evil entity, and are therefore, in themselves, evil. And it's a whole kerfuffle. You could chase your tail on that one for forever. Not that the assumption that all things are good rules out the misuse of such things. Obviously. Humanity is very good at creating double edged swords.
Anyway. With sex being a part of The Plan, it must have a good purpose. So what's the purpose?
I think the biggest, most important purpose is to bring two solitary people together. We are alone, but we crave to be not alone. We *want*, like we want air, to be with someone on a level that is completely open and trusted and free. We yearn to return to that state of original innocence - the communion of persons in a free exchange. Sex, under the right circumstances, helps us bring ourselves, body and soul, together with another person.
Sex is a gift. It is the gift of ourselves, to another person, even as they are giving that same gift (their self) to us. It is the greatest gift that we, as humans, can give to another person. And that's why it's so special. If you're communing on such an intimate level, do you really think that you can do that with multiple people? Really? During a marriage ceremony, the bride and the groom are the actors. It's a sacrament that they confer upon one another. Yes, the priest is there, but more as a witness than anything else. He isn't 'giving' them one to another. They are each giving themselves to the other.
And, even more, the *potential* contained within the act of sex echoes the Creation itself. God, in an ultimate act of Love, Created. Humans, coming together in a communion of their persons, contain the same creation. In an ultimate act of their joint love, they have the potential to create life.