consenting adult 
noun
An adult who willingly agrees to engage in a sexual act: I was the other consenting adult in the affair gay sex between consenting adults will be decriminalized...
- Britain had decriminalized homosexual acts in private between consenting adults in the Sexual Offenses Act 1967.
- They claim that adultery is sex between consenting adults and, so, it is a personal matter.
- What is the basis for the criminalisation of incest between consenting adults?