The purpose I choose to assign my relationships is one of play.
I've tried out other purposes, and they're just not that great.
Play is the thing that people are doing when we think "oh, they're so confident." They're interacting with their environment from stable ground, in the luxurious state of play, free of fear and anxiety. They're in it for their own pleasure, and their pleasure ripples out and infuses others. Confident people are a joy to be around because your interactions with them are play.
The reproductive-sex purpose of marriage is shockingly a beautiful metaphor for this. Two people choose to join their lives together and allow pleasure and play to be the primary creative force in their lives from that moment onward. Pleasure leads to conception, children are best raised and maintained through play.
It's reductive to assume this means that we must engage in heterosexual sex or produce small humans--what it actually symbolizes is that there is a point in life when we may decide that pleasure and play will be the primary force of creation in our life, and lay aside fight, hard work, and struggle.
We can choose to bring forth creation via our various pleasures, and nurture our creations through play.
We can choose to let pleasure and play be the primary purpose of all of our relationships, trusting that beauty will grow from that.
We can establish and occupy a world beyond protection, one that is simply free of threat.