The primary means that the aliens use to harvest sperm from male abductess is through the use of an apparatus with a hose and a cup-like device on the end. This apparatus is placed over the genitals and a suction occurs. This method is fast and somewhat painful.
The secondary method used, the method used when by chance the first method fails is actual sexual activity. This may come as a shock but the aliens will often use a female abductee or even one of the beings they are creating and force them to engage in some form of sexual activity with the male abductee. Often when this is performed a group of aliens will gather and watch, as if they are learning or studying human sexuality. Both the male abductee and the female abductee are mentally controlled during this time. They are not in full control of themselves and are pretty much powerless to resist. This method is very traumatizing to both parties involved.
The third option, the one used with older male abductees, is the dreaded "anal probe", i.e. electroejaculaton. A probe is inserted into the rectum of the male abductee and an electrical charge stimulates the prostate and forces a release of semen.