We know that the Prophet (pbuh) married the ex-wife of his adopted son. What I fail to understand is whether it would look ethical to marry my adopted son's wife? There must be hundreds of ladies at that time then why did the Prophet [pbuh] choose her?
According to the Qur'an the real son is the biological son. Adopted son is not real son. The Qur'an condemned the pre-Islamic concept that the adopted son is just like real son. The people of Pre-Islamic Arabia did not approve of marrying the ex-wives of the adopted sons. The Qur'an corrected this concept. Now we believe that the social construct like the one you are presenting as ethical is not based on moral principles and legal injunctions given by God Almighty. Therefore, it would be perfectly ethical to marry the ex-wife of one's adopted son. There seems to be no valid reason to condemn something as unethical that which the Almighty Himself specifically declared to be ethical.