Birkin and Wesker are different though...
Birkin was married to his wife for a very long time, Wesker was created by Umbrella and was a heartless bastard who only loved himself, he believed he was a god and turned down Excella's romantic advances in RE5, RE6 character who people say is Weskers son it's weird, unless he was having a fling with a lady Umbrella scientist.
^ If you knew anything about Wesker you would know that he was not created by Umbrella. Please read some files about Wesker or play some games other than RE5 with Wesker in it and I'm sure you will figure out that before he injected himself with the prototype virus that gave him powers he was a normal human. Which means he surely had some emotion and could have possibly had a kid with someone. It's not that big of a deal.
Folks, we still don't know if Wesker's so-called son has been concieved in a normal, natural way or not. I think Jake isn't the product of a normal impregnation. Why? Because if Ada is to be believed, he has inherited Wesker's special blood. However, as the person above me already said, Wesker was just a normal human up until that fateful night in 1998, so for his son to have his special blood, said son must have been "created" after 1998 - but that wouldn't match his apparent age in Resident Evil 6. So before we discuss the question whether or not Wesker ever loved a woman enough to make a baby with her (and even that doesn't require love), we should wait for more information detailling the circumstances of Jake's creation. ~ Froufrou
How comes in RE5 - Lost in Nightmares, Spencer mentions information about the Wesker children, to which Wesker replies - Are you saying I was manufactured, if you read Wesker's page it will say he killed his creator Spencer.
Jake could have been created after 1998, maybe he aged faster/quicker like Spencer in RE0, where Billy & Rebecca meet a old guy who quickly becomes young again, it could be like that.