The meaning of the original unity of man can only be understood by knowing man in his entire being.
Man and woman are, in a sense, two 'incarnations' of the same metaphysical solitude before God and the world. They are two different ways of being a 'body' (separate physical forms), but at the same time man, and each of these different ways of being completes the other. You know, the whole, puzzle pieces fitting together sort of thing. Each is complete in and of itself, a whole piece, but placed together, the two form a more complete being.
Femininity finds itself in the presence of masculinity, and masculinity is confirmed through femininity. Again, I fall back to my 'you need contrasts to comprehend a thing'. A man is only a man, when contrasted to the woman. Leaving aside the fact that without both sexes our species would have died out before it became a species (or, I suppose we might somehow find a way to reproduce asexually, like worms, but I assume that's less fun than the way we do it. ;) )
'Precisely the function of sex, which is in a sense, "a constituent part of the person" (not just an "attribute of the person"), proves how deeply man, with all his spiritual solitude, with the never to be repeated uniqueness of his person, is constituted by the body as "he" or "she". The presence of the feminine element, alongside the male element and together with it, signifies an enrichment for man in the whole perspective of his history, including the history of salvation.'
The unity of which Genesis 2:24 speaks is, undoubtedly, expressed and realized in the conjugal act. 'The biblical formulation, extremely concise and simple, indicates sex, femininity and masculinity, as that characteristic of man - male and female - which permits them, when they become "one flesh", to submit their whole humanity to the blessing of fertility.' However, it is not just the physical that is included in the joining of a couple. While physical intimacy is vital and important to a healthy relationship, it is the spiritual, the mental, that truly brings two people together and keeps them together and happy.
One can have a purely physical relationship, based on the 'animal attraction'. It's fun, and it's satisfying, on a certain level. But those relationships do not last. If you want to fall back on the old fire metaphor, it burns hot and fast. And that means it consumes itself quickly, and burns out. But a relationship based on shared ideals, shared faith, on the mind, on the soul, will burn slowly. Smoulder, if one will. And that means it won't burn out.
But the physical *is* important. Every time man and woman come together, they echo back to the act of creation itself. They participate in an act of life. Through sex, we rediscover our own humanity.
Another interesting idea that was raised was that of the cleaving of a husband and a wife together as an act of choice, rather than nature. A person, given that, in order to exist, we are the product of our parents, belongs, by nature, to them. (Not in the sense of property, of course.) However, that child (as an adult) steps outside of their natural place of belonging, and chooses a mate. They choose to start a new life, with a person of their choice. And yes, I realise that, historically, the children didn't choose their spouses, the parents did, but I think we're meant to be speaking of the ideal here. Where one is free to choose their own mate.