The couple tag is not for sex between any two people, etc. But we already have the threesome tag for sex between three people, so why not have a sex tag for two? Stealing the duo tag probably works well for this, because it doesn't get much use.

A good tag would be twosome, if it is even needed.

Why not just tagging both duo and sex, and searching duo sex?

leaving aside what you'd call it, you'd have to tag at least a third of every rating:explicit image on the site. any takers for that job? :V

I always thought that sex was assumed to involve two individuals, although that doesn't seem to be the case at all.

elad said:
leaving aside what you'd call it, you'd have to tag at least a third of every rating:explicit image on the site. any takers for that job? :V

I would assume in this case that an implication would solve that issue, if presented.