Bigotry as I mean it is treating people differently based on their identity rather than their actions.

We all have preconceptions about people and groups of people, some of these based on stereotypes, some on data, and they tend to be largely accurate. I'm not about to treat somebody horribly based on his visible characteristics, but I'm absolutely going to use them to form my initial impression of him. You probably do this all the time when it comes to e.g. age - you treat kids differently than adults, sex - you treat men and women differently when it comes to choosing a partner, and so on. These would be bigotry according to your definition, but I don't think you'd say these things are wrong.

First, two things happening at the same time does not mean one thing is causing the other. How do you know less monogamy isn't causing higher rates of homosexuality, or that they aren't both a result of global warming or something?

The slippery slope of degeneracy is what was predicted at the time that sexual liberation came to the forefront in society. That's why a causal link is far more likely; there are reasons why it was predicted, and then it came to pass, so we have to give some credit to the hypotheses of those old conservatives and reactionaries that opposed it. There's nobody who predicted that global warming would cause less monogamy, so that hypothesis is far less credible.

Second, you don't need to approve of other people's life choices. If you think that monogamy is necessary, go ahead and lead a monogamous lifestyle. That doesn't mean you need to force the rest of the world to emulate your specific preferences.

This is liberalism writ large - a focus on the individual and his choices but without concern for the effects on wider society. I'm not talking about my preferences; I'm talking about the effect the sexual revolution had on the whole of society. Even granting liberal premises, it can still be true that sexual liberation was bad for society even if we have a liberal moral obligation not to oppose gay marriage or extramarital sex.