Joshi daisei: Sex sotsugyô shiken (1982)

A female university student in her third year, about to graduate next year, finds herself in a situation where her boyfriend's recently started business is taking off and keeping him busy, meaning they can't see each other often and their sex life has been on hold for a while. At that time, a female friend tells her that her boyfriend has a mistress contract with a businessman, but the contract is about to expire and he wants to sign a contract with a new woman, so he asks if she would take over.