Wet Girl Yoshiko (1985)

A man who is also engaged becomes worried when he overhears a woman he picked up saying, "I'm engaged, but I'll make a good wife when we get married, so I'm having sex with lots of men while I can." Could his own girlfriend be thinking the same thing? So he asks his best friend, who is also engaged, to try and seduce his girlfriend while he's away on a business trip, and apparently she readily agrees. Now his best friend is worried about his own girlfriend and asks his boyfriend to investigate...