You're the xenobiologist on a spaceship that's studying a lifeform found on one of the planets your team recently visited. It's a mass of tentacles, barely the size of a tennisball, but it's growing quickly and becoming more and more aggressive. It seems to be the most at peace when it's in its hide - one of those little plastic caves you put in a hamster cage - but it will outgrow that, too, by the end of the week. There's really only one place that it can be kept safe that it won't outgrow immediately. If you can't calm the creature, your superiors are going to kill it. You know how to help, and you've grown fond of it, so you volunteer. Your womb will make the perfect warm, dark place.
As you enter your lab, the lifeform goes into a frenzy, slamming itself against the glass walls of its little enclosure. It's grown since the meeting two days ago; now the size of a softball. Your partner closes the laboratory door behind you, and the locks slide into place with a heavy metallic thud. You take a deep breath, slide your pants off and kick them aside. The creature calms as you approach, waiting to see what you're going to do before it makes its move.
"Alright, you," you address it nervously. "I'm going to open your enclosure, and you're not going to try and kill me, okay?"
Its tentacles, transparent and slimy, feel around the seams of the enclosure's door. You sit in your chair, wheel up to the table, then spread your legs. With a shaky breath, you reach for the latch on the box, then flick it open. Instantly, it pushes the door open and wraps its tentacles around your hand. You check your breathing, trying not to panic as it begins to squeeze.
"Easy now..."
With your free hands, you spread your lower lips, then position the lifeform in front of your opening. It was smart and curious. It would understand.
"You'll like it in there," you promise when it hesitates. "No tricks. Just a nice place to sleep."
One tentacle reaches out and curls around your clit, while another prods at your urethra and a third slips into your cunt, feeling, exploring deep. You grit your teeth as the tentacle in your urethra slides in further and pray that the alien won't choose your bladder instead of your womb. It adds two more tentacles to the one in your cunt, and you can feel it exploring your cervix.
It feels good.
But that pleasure turns to pain as it forces your cervix open and pulls itself partially inside of your cunt, stretching your entrance around its soft body. It could relax, making itself smaller, but it wanted to get a good feel for the space inside.
"That's it...." you breathe. "All yours."
The rest of it slides inside, and you can't help but clench around it. It freezes, but only for a moment, and then it begins to force itself past your cervix and into your womb. That stretch isn't nearly as pleasant as the previous, and you grit your teeth hard as you ride it out. The creature is cautious, going slowly, stopping partially inside, forcing your cervix to remain stretched around its form.
And then it slides the rest of the way in and your body closes around it. It shifts and turns and pushes at your walls, bulging your belly outward until it finds a comfortable position and settles down. And there it remains, your new little friend, content in the warm and dark, where it will feed off of your menstrual blood and continue to grow. How big? No one knows, but even if you were to change your mind, there is no way to get the creature out now. You and it are one, from now until it decides its ready for a new home. If it decides its ready for a new home. It may be content to stay with you forever.