In the novel, Emma is best friends with an orphan. Her governess marries a rich, older man and Frank Churchill, rich and handsome, marries a poor governess-to-be. I've always thought that this would be looked down upon at the time, but Austen's book seems to shirk that.
On another note, Emma is very independent and claims to never desire marriage. Was this independence not seen as unusual for that time?
On the side (which I think has more of literary rather than historical answer), Emma is not being entirely sincere when claims to never desire marriage. Rather she is covering for the fact that her father's happiness is dependent on her never marrying in his lifetime (which she wishes to be quite long). Her marrying (unless he were scandalously beneath her socially) would almost certainly mean her moving into her husband's household. She is thinking as a loving daughter with these remarks rather than being sincere to her own personal feelings.