I think of it as spiritual, or as spiritual as something can be. It is physically the closest you can be to another person. So many connections are necessary for it to happen: movement, breathing, penetration (or not). All the senses are engaged, and it allows a full and complete experience of the other person.
It is an opportunity to make something feel spectacular and desired, and a chance to feel the same. It's a combination of extreme selfishness and extreme selflessness that nothing else can achieve. Sex tells you so much about a person, more than you might be able to learn through words.