Swingers Massacre (1975) - Originally titled "Inside Amy" this strange little film looking at the psychological effects of on a couple who start spouse trading in a swingers club will get your attention. Charlie Tishman (James R. Sweeney) is a married and bored lawyer who is looking to spice up his marriage. Sadly we see from the brief, oh so brief, love making session with his wife Amy (Joyanne Mitchell) that there is no satisfaction in her love life either. When Charlie suggests that they start going to a swingers club to seek alternative mates, Amy is reluctant but knowing Charlie cums in 20 seconds she realizes she might actually get something out of it. Everyone is sweaty and horny at the swingers club and it is not long before the couple are invited to a private party where partners will be switched. Amy is still reluctant why Charlie is glaring at the other women and seems more open to the idea.
Going to the private party we see that poor Charlie can't seem to get it up with the attractive wives of the other men. Counter that with Amy who seems to love every advance and performs with any of the men that approached her. I think this is where the original title Inside Amy comes from where for the first time she is experiencing the effects of good sex. Her development is the change in her character where she realizes that good sex is possible. Not the vulgar interpretation that she has every possible man inside her.
Really what going to the swingers party does is highlight the inadequacies of Charlie while allowing Amy to be satisfied for the first time since they got married. He is embarrassed by his lack of performance and hits the bottle instead of trying to deal with his issues. He hears the comments about how good his wife is. He is ashamed by the encouraging comments that he can try again. Like in his marriage when he fails to perform for his wife he rolls over giving up; here at the party he hits the bottle drinking himself into a stupor that completely negates the chance of him rising to the occasion. He effectively makes the problem worse. Let's say that when he could not satisfy Amy at home, instead of ignoring the problem he had spent some time trying to digitally stimulate her instead of the more extravagant idea of becoming swingers. He and she could be closer and more involved, but he does not want to confront his issues. Instead the stimulus of swinging seems more alluring instead of say wearing too condoms so he won't cum too fast when with Amy. It is a sad result for a poor plan that drives Charlie just a bit insane.
This is a story that takes place before the first great slasher films were made. So Charlie becomes more of the crazed mentally unstable killer than the slasher killer. A Norman Bates type with a psyche broken from the humiliation of his wife's pleasure at the hands of others. Charlie cracks and suddenly he is looking to kill anyone who has been with Amy. As the bodies pile up the police begin putting together that there is a group of swinging couples and that they are the ones being killed. Since everyone goes by first names their task is not easily but eventually they get Charlie the lawyer they know at the court house. As they close in on him, he is confronting Amy in what will become the his final climax.