Atheism

I agree that religious beliefa are generally on the decline, but that's not objectively a good or bad thing.

The word virtue unfortunately has too many meanings in this context:

However virtue, if defined as moral excellence, simply does not exist in a fully objective sense. The best definition I can try to personally approximate is to "choose your own actions in such a way as to minimize negative and maximize positive effects on the people they affect". If I use that definition to reason about the virtuousness of being sexually liberal, I cannot see it as anything other than a virtue, other than hurting others by e.g. being promiscuos within a relationship.

On a more general note, monogamy was not always the norm. Historically a lot of cultures have thrived under polygamy or similar systems. The most well-known one (assuming you're from North America) are the Mormons, which accept and encourage polygyny.