It's said that the male aliens can reproduce with the female humans, barring some issues with size. The alien offspring is a little larger than human females are built to deal with, but many times they can be delivered successfully.

The issue with female aliens and human males not being able to reproduce is mostly unknown, but it's assumed that it also has to do with size/potency. The human male's sperm is not potent enough, and the offspring would be too small. The alien male's sperm is apparently... very strong.