I'm told of a species of sea slug — Siphopteron quadrispinsom, if you really want to get fancy about it — that has both male and female organs, and so can have all kinds of kinky, painful sex, including the kind of foursome that involves only two creatures. The thing about these sea slugs, though, is that the reproductive, male-to-female version of sex is extremely painful for female slugs, so painful that, in theory, slugs should play female just as long as it takes to accrue a little storehouse of sperm. In fact, however, slugs enjoy their heteronormative liaisons much more often than that.
According to the New Scientist, this particular type of sea slug, when playing male, wield its rapier-penis with all the thrusting gusto of a musketeer, or a doctor giving a tetanus shot — the male slug stabs its partner, anchors the penetrating organ in place with spines so that it can inject a whole thimble-full of prostate fluid into said partner. This mating trial is apparently so traumatic for the "female" slug, that slugs prefer to act male, and very often resist doing it altogether.
However, Rolanda Lange of the University of Tübingen in Germany has found that the slugs act female more often than they'd need if they were simply trying to ensure that their eggs were fertilized. Lange and her colleagues captured different groups of slugs and found that the slugs that produced the most eggs acted female at a medium-rate. Though it would seem reasonable that slugs would only want to be female for the very bare minimum of progeny-creating time, Lange and her team found that slugs were female much more often than that, meaning that traumatic male-female sex has some other inscrutable benefit to the slugs that scientists just haven't uncovered yet.
A researcher at the U.K.'s University of Sheffield thinks it might have something to do with extra nutrients in the injected prostate fluid, but, more likely, it probably has something to do with the E.L. James saga mainstreaming BDSM among sea slugs, who try some bondage stuff out on each other because it's like a huge pop culture phenomenon even though they don't really like bondage at all and are just doing it to be cool and make new sex friends. If only all the slugs would be honest with each other about their sexual proclivities and not let pop culture fantasies determine what they do with their genitals, we might have a more placid, painless undersea environment, which would assuredly lead to more choreographed musical theater à la The Little Mermaid.
Slugs benefit from getting stabbed while having sex [New Scientist]