For example, when state laws supporting segregation and anti-gay marriage become unconstitutional, is it the interpretation of the US Constitution that has changed or is it socio-cultural changes within a society that allows judges to rule in line with Constitution?
The Constitution of the United States is considered a living document. This is due to Supreme Court rulings as you have mentioned on top of an ammendment process that albeit slow could theoretically allow the entire constitution to be re written. Also the framers of the Constitution worded the document so that many things may be interpreted loosely. There was an argument over how loosely it should be interpreted between those who supported a strong and weak federal government. Hamilton and Jefferson were famous for arguing over this. At the same time the last ammendment occurred last century so I would say it is somewhat alive however there do not seem to be any drastic upcoming changes to it such as a new ammendment.