well no there isn't a reference because officially any kind of illicit sex was contrary to Christian morality and that was the official morality of ENgland at the time. And for much of JA's world, of country squires and parsons, they probalby DID stick more to the tenets of Christian morality (but then again Harriet Smith's father is a merchant!)
Still, Regency and pre Regency high society (ie Willoughby's world) was frequently lax in matters of sexual morality.. It depends what you call a cad... but many of the aristocracy did have mistresses and there were "unoffically" rules for how this was done. It did mean that certain categories of women were considered "OK" for seduction... wheras others were not. ie single girls of a certain rank were "forbidden fruit" but they could be seduced once married... provided it was all done discreetly....
With regard to your point about people knowing about Eliza, most people who had met her or heard of her problaby knew the gossip that she was Col Brandon's natural daughter.. so she was already someone with a dubious background....