That theory didn't originate from the autopsy, but rather from laboratory tests on Foster's clothing. There were semen stains on his underwear and blonde hairs on his clothing.
But there is an alternative explanation for that. Ejection of semen immediately after death is fairly common in men, and doesn't have to be caused by a voluntary ejaculation in life. The precise reason for this is not well understood, but it is known to happen.
As for the blonde hairs on the clothing, they could have been shed by an attacker in close proximity or contact.
Informative. Thank you.