Used by everyone from galactic wanderers, demigods, space kings and queens and even frost giants, these cosmic condoms are large and stretchy enough to accomodate even the most well-endowed world devourer, and collapsable and compact enough to comfortably fit even tiny astronauts like in that one Twilight Zone episode.
Not every alien uses this crucial life-saving (and life-preventing) device, such as Space Pirates like Xenu and of course horrible rapist aliens like the Greys. These unethical sluts are putting themselves and all others at risk of urethra-bursting xenomorphs and fiery balls of AIDs from space. Do not believe any smooth-talking alien (such as Zaphod Beeblebrox) who tries to convince you that 'this will feel better' or 'I'm clean, baby, I just got tested on Tantive 9' or 'hey, it's okay, you can't get pregnant that way.' Trust the experts, you don't know how super-effective an alien could impregnante you anywhere.
Long ago, early humans discovered these Intergalactic Prophylactics lying around everwhere on the sun-scorched African plains, and mistakenly thought they were merely bounce reflector discs for use in photography and film lighting. Only recently have archaeologists, scientists, astronomers and historians all completely agreed that these are clear evidence of ancient aliens.