I think we'd have to talk about "crime" first. Society can make anything a crime ... at least in the short run. If the society declared adultery a crime, then it was a crime. However, if you give crime a more subjective interpretation, if you think something is a crime because YOU think it's a crime, then it's more complicated. I think Hester understood that her action was a legal crime and that she had to bear the consequences of her action. I also think that she felt that what she and Dimmesdale did had a "consecration" all its own. Yet, earlier in the novel, Hawthorne remarks. "She knew that her deed had been evil; she could have no faith, therefore, that its result would be for good." Perhaps her ambiguity matches our own?
Our attitude toward adultery today is interesting. Although people seem to indicate that they are "cool" with it, that it's just a part of life, the person cheated on rarely takes such a cavalier attitude toward adultery. Are we a better society because nothing seems to be a "crime" any more? While we probably should not be destroyed by our mistakes as Dimmesdale was, might we not be a healthier society if we accepted, as Hester does, her responsibility that comes from living as a member of a society?
To address your first question, I think a changing and expanding world made adultery seem less important. When your social circles are small and family is all that is there to support you - dishonesty is traumatic. But in the world today, where people thrive on indepedence and have many opportunties to survive financially, the consequences of a marriage disaster don't have as far-reaching consequences. (or, at least, so it seems)
While we still have puritanical values and behaviors in America today, another reason Hawthorne got into this story at all was his disdain for puritanism, havign been raised by a faher that still held puritanical views. It helps to underscore how unforgiving the society was, and how unfair it is to treat two people in love.
A pragmatic reason to approach the story was that a similar event is said to have happened in puritan times, within Hawthorne's family, and I personally think that it was a chance for him (our author) to air his family's dirty laundry as another dig at his father's choice of religion, Puritanism.
I include these two points merely to suggest that Hawthorne might not have commenting on society or other social issues at all.
For me, I feel that Hawthorne felt that adultery was less a crime than personal choice. The way he depicted Hester and Dimmesdale as tortured and unable to be together makes it heartbreaking. These two were in love, not just carried away by lust.
As for the social acceptance, I think that is our culture. The evolution of sexuality has just naturally brought us to the point to where we are rarely shocked by adultery. While no one would like to be cheated on, the public doesn't do more than gossip about a persons philandering ways, instead of the public humiliation we saw in the novel.