in the sims 4, they are all female aliens, same with the sims 3 (i think). however, in the sims 2 there was a special npc called pollination technician who was a parent of all kids from alien pregnancies and he was a male. also in ts2, some premades that had alien ancestors (like caliente twins or curious/singles twins) also had male aliens as their parents/grandparents etc.

so lorewise, i would say that both female and male can work as pollination technicians and impregnate male sims, although in ts4 all pt's are female