Answer
Salam Dear Ayesha,
Thank you for your questions and for contacting Ask About Islam.
I hope my answers will quench your thirst for a bright and unadulterated image of your beloved Prophet, peace and blessings be upon him, and his household.
As I mentioned before in an answer to a similar question the enemies of Islam has always been working on the Prophet’s private life as being the Achilles’s heel of our religion that we have to be ashamed of and keep defending or rather apologizing for throughout our lives!
To some extent I, like you, am not convinced with this only explanation that the Prophet married 9 simultaneous wives in order to help them out because, as you said, he could have ordered this help to be given to them by his companions.
To me, the source of confusion in understanding this issue of the Prophet’s marriages is that we judge a 7th century style of life with a 21st century criterion. At the Prophet’s time and environment – Arab Peninsula – it was a normal thing for a man to marry an unlimited number of wives plus taking an unlimited number of women slaves. Then, when Islam was revealed, it restricted the number to only four wives plus encouraging people to free slaves.
All Muslims had to abide with this divine command. Thus, they kept only four wives and divorced the rest. The situation for the Prophet was quite different as he was the only one allowed to keep his marriages.
This was for one simple reason that his wives were not allowed to marry after him. Therefore, it would be inhuman for those noble ladies to stay the rest of their lives unmarried, while most of them were in their teenage, twenties or thirties.
It is true that each of those marriages was for a certain reason, either to honor a friend by marrying his daughter – in case of `A’isha and Hafsa – or to help a wretched woman… etc. But he didn’t leave this task to his followers as you have suggested because nothing of the norms and conventions of his time actually prevented him from doing it himself. He was behaving normally according to the established lifestyle of his age and environment.
Again, the way he married a 9-year-old girl was not out of the norms of both time and place, specially when you know that the girls of this region – desert – till now reach their puberty in an early time – ni
ne and ten years old. So it was not an act of harassment to Aisha or violation of her innocence, but it was a norm that would put a girl to shame if she waited any longer after this age without getting married.
Please notice that her sister Asmaa was married to Az-Zubayr at the age of eleven, and so did most of the girls at that time without even considering the age gap between the bride and groom.
Actually, there was a need for the Prophet to marry a girl at this age, in order to live long after his death to teach people lessons from his private life that only a wife would have witnessed. Thus, his many marriages and his marriage to a young girl were not something weird or to be ashamed of or even apologized for, from the criterion of his time as it may be now.
As for your question concerning marrying the wife of his adopted – and not step – son, in fact it was not his decision but a command from Allah that was given to him in the Quran when it reads the meaning of:
{And when you said to him to whom Allah had shown favor and to whom you had shown a favor: Keep your wife to yourself and be careful of [your duty to] Allah; and you concealed in your soul what Allah would bring to light, and you feared men, and Allah had a greater right that you should fear Him. But when Zaid had accomplished his want of her, We gave her to you as a wife, so that there should be no difficulty for the believers in respect of the wives of their adopted sons, when they have accomplished their want of them; and Allah’s command shall be performed.} (Al-Ahzab 33:37)
It was for a wisdom which was to nullify a pre- Islamic norm that the adopted sons were to be treated like natural sons. Allah wanted to teach His servants that adoption is prohibited in Islam.
Thus, those formerly adopted sons are but strangers to their former adopting fathers. In order to stress this message Allah commanded His Prophet to marry noble Lady Zainab, the divorcee of his former adopted son.
It is worth mentioning that the Prophet was not behind her divorce to take her for himself but it was a natural divorce. It is also worth mentioning that the Prophet was embarrassed to carry out the order of this marriage as he was afraid of people’s reaction. So Allah Almighty, as appears in the verse, rebukes him for fearing the public opinion concerning a command given to him by his God. 
I recommend Ayesha that you only take your knowledge from original Islamic books and not from books that spread hostility and misconceptions about Islamic facts, specially those concerning the Prophet, peace and blessings be upon him and his revered household.
Thank you again and please don’t hesitate to contact us whenever you have any questions “bothering you” and always keep in touch!
Salam.