Aliens may not have a binary sex distinction. Perhaps there are 7 sexes, each pair having difference success-rates for producing offspring. Perhaps 3 or more individuals need to pool their genes to produce offspring, unlike our 2: a creampie gangbang as essential requirement. Perhaps they create new individuals by assembling them according to specifications, requiring no sex at all. Aliens may be very alien....

Given the time needed for interstellar travel (barring highly relativistic speeds, warpdrives, wormholes, and cryogenic hibernation) it seems helpful to have procreative ability: you may not arrive, but your great-grandchildren will.

Put down the glue lass...

The scary part is if they send out a crew like BELLA!'s minions.

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Unidentified fucking onlookers!