Did God have different standards of purity through the Old Testament?

Something I've always wondered as I read through the Old Testament, and this was brought to light tonight in Genesis 17-19's accounts of Sodom and Gomorrah and then Lot and his daughters, is the different standards people had in regards to sexual habits or practices. We see time and time again throughout the Old Testament things as simple as polygamy, to things as seemingly off-the-wall as Lot's impregnating his own daughters (albeit not while of sober mind). Similar stories abound throughout the stories of Jacob and his sons to the kings of Israel and Judah. We haven't gotten as far yet to read the Mosaic Law concerning sexual purity, but judging from the practices in Jesus' day it seemed pretty strict with severe punishments. But at the same time, people like David and Solomon had multiple wives and the subject is never really brought up as being in conflict with the Law (or rarely even to God's disfavor) except for when David killed a man to get to Bathsheeba.

On the other hand, God destroyed the cities of the valleys of Sodom and Gomorrah because of their wickedness, and has largely been attributed to their sexual deviance as shown in Sodom's treatment of Lot's visitors immediately prior to the city's destruction. However, Scripture does not say it was due explicitly to sexual behavior - just their wickedness, generally. Is the sexual deviance-cause a construct of modern Christian culture, which tends to view sexual sin as the worst possible abomination imaginable? [I am in no way downplaying the nature of sexual sin, as it is sin. Sin separates us from God and therefore all sin should be avoided like the plague.]

Joshua had mentioned recently that he was listening to an audio series about the "Emergent Church", which I only caught bits and pieces of through my conversations with him, that apparent different standards seen throughout the Bible has to be seen through the eyes of context, but I'm not sure what exactly that entails, and whether this seeming contradiction might be an element of that changing context. And there was also something about the need for context and human-wrought "logic" being a construct of modern philosophy. I don't quite know what I'm getting in to here, so I should stop before saying something ignorant. Anyway, this whole post is just an observation. Now perhaps it will spark some debate...?