its nothing to to with male blood lines at all. It is to do with genetic inbreeding. Adoption does exist in Islam but isn't mis-named. You can take in and care for an adopted! child but he or she retains their name and identity (and technically a non related member of that family unit along the line can freely marry as there is no chance of genetic deformity etc. Its a nit picking issue (as with most of what you discuss here) but it makes sense in those one in a million cases
As for marrying your fathers former wife or mothers former husband. Get a life!
Here's Islamic genetics & Islam's superior knowledge of genetics doubtfool!
Fatima married her first cousin, Ali. Inbreeding has a higher chance of causing genetic abnormalities, especially consistent inbreeding. By allowing Fatima to marry Ali, Mohammed has made it seem perfectly fine (possibly even encouraged) first cousin marriage.
Problems in Saudi due to inbreeding-
http://query.nytimes.com/gst/fullpage.html?sec=health&res=980DE0DF1F3DF932A35756C0A9659C8B63Problems in Pakistan
http://news.bbc.co.uk/1/hi/programmes/newsnight/4442010.stmIt is ironic that first cousin marriages are ok yet those who have been suckled by the same person are not allowed to marry-
Sahih Bukhari Volume 3, Book 48, Number 828:
Narrated 'Uqba bin Al-Harith:
I married a woman and later on a woman came and said, "I suckled you both." So, I went to the Prophet (to ask him about it). He said,
"How can you (keep her as a wife) when it has been said (that you were foster brother and sister)? Leave (divorce) her. "
Its possible to tell an adopted son or daughter that he\she is adopted without marrying their ex spouse, you know!
Marrying their ex spouse will not reduce the risks of any inbreeding!
Telling them that they're adopted is good enough!
In case of both father\mother's ex spouse & adopted children's ex spouse, there's often no blood relation involved, but such an act is rightly considered vulgar by most societies even today.
There was no reason for Muhammad to strongly condemn one but want to so encourage another that he had to marry his ex daughter in law!
Its your attempts at apologetics that needs to improve.