means your lovemaking skills leave much to be desired, you are not a bedroom bully i.e., a one-pump champ who can hardly last long enough to satisfy their partner for them to reach an orgasm (kutunda). This may either be an effect of being overly excited, high levels of intoxication, or ED (erectile dysfunction aka impotence) particularly caused by spinal problems.
Musana in chiShona language means back or spine whilst derere means Abelmoschus esculentus (okra, ochro, or okro). In other words, the phrase musana wederere is a metonymy to say your sexual dysfunction (i.e., a spine that is as weak as an okra stem) is affecting your sex life since you can't perform really well (kugutsa munhu).
Therefore it goes without saying if you have musana wederere polygamy isn't right for you. There is nothing wrong with being celibate either. It's fine. It's not the end of the world — the beat goes on, just saying.