Take a group of women who are not in monogamous relationships and who are having sex with men who have other partners. Randomly assign half to group A and half to group B. Take one partner for each woman. Instruct the partners of the women in group A to not have sex with any other women for two weeks, and instruct the partners of the women in group B to have sex with their other partners frequently for two weeks. Ask the women to self-report how pleasurable they find the sex, and how often and powerfully they orgasm. Tell everyone participating in the study about this procedure, and get their consent to it.
Take a group of women who are not in monogamous relationships and who are having sex with men who have other partners. Randomly assign half to group A and half to group B. Take one partner for each woman. Instruct the partners of the women in group A to not have sex with any other women for two weeks, and instruct the partners of the women in group B to have sex with their other partners frequently for two weeks. Ask the women to self-report how pleasurable they find the sex, and how often and powerfully they orgasm. Tell everyone participating in the study about this procedure, and get their consent to it.
It seems like finding a statistically useful number of such scientifically-inclined, polyamorous couples would be quite a challenge.
To be clear, they don’t have to be polyamorous couples: just using women who have a friend with benefits would work.