consensual (adj.)

1754, "having to do with consent, formed by consent, depending upon consent," from stem of Latin consensus "agreement, accord" (past participle of consentire; see consent (v.)) + -al (1).

Until modern times used almost exclusively with reference to legal contracts and to the eyes working together reflexively; its sense was extended in the language of sociology and psychology from 1950s (of social groups, non-arranged marriages, etc.) and in the legal discussions of rape and other sex crimes by 1977.