I've always thought it was a little strange that God doesn't ask the serpent for his version of what happened. Maybe God was tired of hearing people pass the buck, or perhaps since the serpent is apparently Satanic, He's just not interested in hearing any lies at this point in time. Anyway, after asking questions of the man and the woman, He just goes straight to pronouncing the curse on the snake.
Now a part of this is definitely my own opinion, and I'm not sure whether more formal theologians share this viewpoint, but I don't think God has much of a choice in this matter. A lot of Christians seem to operate under the assumption that God makes up the moral law, and that apart from God creating moral law, it simply wouldn't exist. I tend to believe that there may be certain aspects of the rules of morality and spirituality that God has no control over. I know many skeptics would ask whether that means God is not omnipotent after all.
I think omnipotence, while still being omnipotence, is actually limited by logic. I believe there are certain things that are a part of the world that are that way simply because having them not be that way would be absurd. The concept of 1+1=2 is a concept so basic that I don't think even God can make it not be true, and I don't think it's reasonable to expect Him to have that sort of power. Mathematics, geometry, logic and certain aspects of physics are, I believe, beyond God's control: they simply are true. If that's the case (and I leave it without proof as such proof may be well beyond my ability) then might it not also be true that at least some aspects of moral and spiritual law are beyond God's ability to change?
God is a being that is entirely good. Interestingly enough, that creates a difficulty for God, as He is by His very nature repellant to evil. Once the fruit had been eaten, God was unable to have the same sort of relationship with mankind that He'd had before. As I said previously, Adam "died" to his relationship with God. This in itself was a curse, as having an intimate relationship with God is (so a Christian believes) the absolute best thing one can have. So part of the curse that now comes upon Adam, his wife, the serpent, and the whole world is actually not by God's choice, but simply the logical conclusion of God having to step back and let Adam live without Him. Some of it is by God's choice, either to punish, to fix the situation, or both.
The curse God gives the serpent in verse 14 seems to be clearly punitive. Essentially, as just about everyone reads this, God takes away the serpent's legs, forcing it to walk on its belly and "eat dust". I don't see any reason to take that latter concept more literally than the fact that, having its face always on the ground or mere inches from the ground, it probably does swallow a fair amount of dust on a regular basis. (Note that this serpent takes on a curse that eventually affects the roughly 3,000 modernly known species of snakes. Perhaps all those species evolved from this one and its mate?) The part of the curse in verse 15 is part natural and part, well, something else. Simply because of the role that the serpent played in all of this, and because of the form snakes took afterwards, they became despised by humanity. (In general, of course; I've owned pet snakes myself, and think they're quite nice.) But of course, many Christians have also taken this verse to be a prophecy of the crucifixion: that one day a man would come who would be injured by Satan, but would, in the injuring, cause Satan utter destruction.
God then turns to the woman, and tells her that she is to be cursed with painful childbirth and an apparently subordinate position under men. I once read an interesting analysis of this concept from an evolutionary point of view. Of all the animals, humans seem to have the most painful process of childbirth. Interestingly enough, this is due in part to the fact that the pelvis is shaped differently due to our being bipedal, and our infants having very large heads. Thus when we, as a species, were separated from other animals, it was when we walked upright and became intelligent which led to change in social structure, development of tools and language, changed sexual practice, and difficult childbirth. Whatever your views on this, I think it's true that being intelligent can be painful at times. The woman wanted to be smarter, and it cost her, whatever the reason was. The fact that she ends up in a subordinate position to men is, in my opinion, not because God wanted to push her down, but because men, in their fallen state, have a tendency to do this anyway, which doesn't make it right. Maybe I'm too much of a feminist to be a good Christian.
Then God turns to man and curses the ground. (The SAB notes that Adam is cursed for listening to his wife, but I don't this is a sexist thing; only that Adam should have listened to God rather than anyone else at all regardless of gender.) Adam now has to work to get his food, rather than having lovely fruits to eat in a beautiful garden. Whether new types of plants with thorns and such came about because of this curse, or that they simply didn't exist in the garden, and man had to deal with them when he was tossed out, I don't know. I think there was a bit of both. The garden was meant to be an ideal situation, and probably had only fruit trees, and perhaps soft grass underfoot. Outside, things were wilder, and work was needed to produce something edible for mankind. And of course, after working hard for years and years to make things comfortable for himself, in the end physical death would come.
Now, natural consequences or choice of God, why are these curses a good thing? As I said above, the assumption we work with here is that the relationship that mankind had with God before the fall is the highest good that man can enjoy, but eating the fruit has destroyed that good. If God is righteous, and by His very nature that He cannot change, demands righteousness, how can an unrighteous human being come to have a relationship with God? The answer is justice. Man must pay for his crimes against God, and the payment is death. God doesn't allow them to live forever separated from Him, but causes them to live in a world that is difficult and deadly, so that they will always remember that their aim is to make it through life to a better life with God after they pass away. The full details of this really come out in the life of Jesus, of course, but some of it will be in the rest of this chapter.