“If there is a physical body, there is also a spiritual body. Thus it is written ‘The first man, Adam, became a living being’; the last Adam became a life-giving spirit. But it is not the spiritual that is first, but the physical, and then the spiritual. The first man was from the earth, a man of dust; the second man is from heaven” (1 Cor. 15:44-47 NRSV).
I have noticed a pattern starting with Cain and Abel, then Ishmael and Isaac, then Esau and Jacob, then Leah and Rachel, then Mannasseh and Ephraim that God favored the second even though it was traditional in those times to favor the firstborn. Is this pattern connected to “the first man Adam was made a living soul; the last Adam was made a quickening spirit?” Many thanks.
We know that Corinth, while perhaps Paul's favorite city, was also a center of many different cults and beliefs. The fledgling church founded by the apostle was constantly being tempted into compromising his teachings so as to embrace other beliefs as well; it was a source of constant concern for Paul.
Many of those cultic beliefs centered on physical pleasure – but elevated spiritually to a more intense and pleasurable dimension. In other words, spiritual belief was thought to enhance and intensify physical pleasure. I think what Paul is saying here is that they've got it backwards. “It is not the spiritual that is first, but the physical, and then the spiritual.” He does this by relating Adam – the first physical man – to Jesus – the first man to allow his spiritual nature to lift him above physical limitations. In Paul's understanding, spiritual awareness does not change the physical, but rather lifts us above the physical, so that we leave its limited pleasures behind for the sake of greater pleasures in Spirit. I'm not sure Jesus would totally agree, actually – his teaching us to be “in the world but not of it” would seem to suggest that the physical and spiritually are meant to coexist; it's not about leaving one behind for the sake of the other.
Your point about the divine preference for the second of two brothers or sisters is excellent. In each of the instances you cite, it is the second-born who seems just innately more attuned to Spirit. I'm not sure if the implication is that the second-born is favored – both Joseph in the Hebrew Bible and Jesus himself come to mind as exceptions to that possibility. Perhaps it's just to emphasize that the ways of God can never be predicted by, or limited within, the rules and understandings of mortal minds.