I love the idea of being changed by some alien grubs/parasites/other small freaky little things
(I got a lot of examples, but here's one)
Like being a explorer on an alien planet and just this big glowing alien bug, like some of bee or wasp type deal, filling some guy ass with a huge load of eggs and glowing blue cum, just repeatedly over and over, until the guy left a little bloated from how much eggs were filled in them. A cosmic storm hits when they return to the ship so they can't leave and get medical attention, so they just have to wait for it to pass...
But something with the alien biology changing him....his chest is getting fuller, glowy, big glowing boobs of flesh leaking glowing milk. Looking in the mirror they see their eyes changing color, become the same glowing blue. They can feel their stomach getting bigger...every day bigger and bigger, but they can feel something wrong.
Everyday they jerk off and off, seeing as their cock changes, becomes more alien, becoming like the dick that fucked him, an ovipositor, and it stopped shooting human cum, it was shooting glowing cum, and soon, eggs. But it wasn't long till another thing changed, another new home appeared, a cunt for all the eggs to saftley cum out of, and a new home to fertilize~
All the while they became more bug, wings growing, an abdomen popping out, antennas to hear them, the rest of the alien bugs. He was just jerking away, covering his space ship in the alien cum, laying alien grubs all around, and covering the place in eggs, the pod set to be perfect temp to incubate
Perhaps a space ship arrives to answer the sos, and he gets a new person to impregnate, impregnate with his new ovipositor.
And maybe come bring back to the hive, where they can all be happy being the newest addition to the alien bug hive~
Yaaaaay everyone gets to have a big titty salmacian bug tf before having weird nasty bugsex and spew grubs and eggs and disproportionately large sperm
