In Japan, orc = pig-person. (Hence, the overt references to pig-like behaviors, with regular orcs having curly pig tails, and high orcs being boars that charge.)
In Japan, calling a woman a "pig" ("buta") is generally synonymous with extreme masochism/sex slavery (specifically, calling them pigs is like calling a woman "your bitch").
Hence, the "joke" is that they're fierce sadistic monsters that are all extreme masochists as soon as they are in a relationship.
