It's unknown where they came from, but the Earth inexplicably became plagued with isolated incidents. Some kind of new arthropod was made known to the public when adult women went missing. Resembling giant green caterpillars, these big bugs hunt down an adult female. Wherein they will open up the front flaps of their exoskeleton and trap her inside. Closed up, all that's left out is the woman's front torso and groin area, as well as the lower half of her head, sticking out from the underside of the bug's head. In a matter of minutes, the concealed portions of the victim are dissolved and fused with the bug's internal anatomy, whilst the parts that remained bare are still fully functional, and under the control of the bug. However, the vagina, breasts and lips will expand in size twofold. At this point, the victim is considered deceased, are what remains of their body on the front of the creature belong to it now.
If that wasn't weird enough, it's only the first half of the process. After this, the bug will secret pheromones from it's new human parts that will attract an adult male to it. This is in an effort to have sex with the human male, and make him her mate for life. Once in coitus, the bug will imprint on the male, so that it mates with him and never leaves his side. This has caused many ethical issues, like where wives that are taken are now screwing some random guy. But, there is nothing that can be done, as the bug is overly protective of her mate.
Whilst retaining the ability to utter sounds in the victim's voice, the bugs display no signs of further intelligence. Investigations into where they came from and how they evolved this way are ongoing, yet the world is divided on what to do with them. Some say to let them live with their new mate, since they were once partially human. Others say to make the species extinct, to stop any further negative developments. Regardless, these bugs, dubbed Fuckbugs, are just in the beginning of intruding on human society, and it's unknown where it can lead to.