Mainline Protestants were always made up of Cafeteria Christians, so no explanation is needed for them. They seem to find environmentalism, homosexuality, and abortion make excellent fashion statements.
Good on Evangelicals for rejecting liberal idiocy which goes against the fundamental tenants of being a Christian. Good on Evangelicals for being the most likely to reject Democrat nonsense. Good on Evangelicals for having the Protestant Work Ethic which created man's greatest creation, capitalism.
“Why isn’t the Catholic Church excommunicating members who espouse deviant practices?”
Probably for the same reason that a lot of Protestant churches around here are afraid to touch anything remotely controversial: they’re afraid of losing members.
They’d rather pay lip service to the truth and keep deviants around, whether it’s to make themselves feel better that they’re “reaching” these lost souls (when nothing could be further from the truth), to appear fashionably in-step with popular culture as you said, or, most likely, to take in their donation money. We’ve got church leaders of all stripes measuring the effectiveness of their denominations by worldly standards, and that’s a shame.
The young people in the Catholic Church are choosing to remain chaste for the most part.