On Earth, there is a species of lizard that is entirely female (lesbianism in nature lol). They reproduce basically by cloning themselves. So you don't need a male for a species to survive. Although it cuts down on the genetic diversity.
Of course, the aliens are different as they obtain their dna from hosts (as has been pointed out). The only other organism that can compare is viruses, who can obtain dna from hosts and transfer them to other hosts.
