For me dub-con involves an impairment by one of the participants such as if someone is drunk, high or otherwise compromised so they might not do what they would normally do, and has sex with someone they might not normally consider. IE a drunk straight guy being seduced by another guy, I consider it dub-con. I also consider dub-con if a minor is involved. Basically, I guess whenever there is an issue of consent. It isn't always as cut and dried as someone saying 'no'.