Then just create some random sort of alien and tell them they use osmosis :-D.
There is a p&p game called "Earthdawn". There you can play a sort of Stonepeople who just do not have a gender because they do not need it. they reproduce by merging out of stone if they want to
This is a nice solution and prevents the players to notorically try to mate with everything they encounter (they did not call Mass Effect 3 "Mass Errect" for nothing I think the overwhelming matingcontent in the latest Bioware games ruined these games ).
I trust on jack's artist to bring up something weird to widen people's field of vision to ease alien envasion in the future :-D.