The following is according to my understanding and knowledge which could be faulty.
WivesThere are traditions attributed to Lady Aisha saying that she and other wives of the Prophet used to be amazed at the strength of the Prophet, when he used to have marital relations with all his wives in one night. I don't know about the reliability of the narration, it is possible that it would have been created by some later generation person in order to increase the status of the Prophet (from their point of view). Whether true or not, it doesn't really matter, because in Islam it is one of the most important responsibilities of husbands to fulfill the marital needs of their wives and the same goes for wives regarding their husbands. It is a very acceptable reason for divorce if the spouse is unable to fulfill the marital needs. Therefore, the Prophet was also fulfilling his responsibilities as a husband.
SlavesFirst of all, there is nothing in Islam that says this is a recommended or desired practice. In Islam most, if not all, laws are based on creating more good in the society as compared to the possible harm. This concept is called the risk-benefit ration, i.e. whether the potential risk of an action is more than its potential benefit. Out of several different methods through which a person could have been made a slave, before Islam, only two methods: (1) through war (in which case from what I've read, the Prophet only took slaves when the enemy was also in the practice of doing so, this was possibly done in order to deter the enemy) and (2) being born to slave parents.
According to what historical records tell us, slavery was deeply integrated into the society, slaves used to work on the crops, took the caravans, pretty much did all the work. In such a case, it would have been extremely difficult and harmful to remove slavery overnight. In our modern society, a comparison can be drawn with a country trying to get rid of the system of interest in the economy. Therefore, it had to be removed gradually. When a female slave bore the child of her master, it automatically meant that she could no longer be sold (or transferred) to anyone else, and additionally, on the death of her master she would become free. Along with integrating the practice of freeing slaves as expiation of most, if not all, major sins, this was another method to eventually decrease slavery in the society. Furthermore, these children had the same rights as other children of the master, therefore they were able to lead good lives. As rich families used to have more slaves, these children grew up in high status homes and from Islamic history, we find that many times they ended up being commanders of Muslim armies, ambassadors, governors and sometimes, if I I'm not mistaken, even kings. Some scholars also give the reason that this allowance was also a method for decreasing the possibility of slave women going into prostitution to fulfill their needs.
Unfortunately, the later generations of Muslims misused these allowances and used them as loopholes to fulfill their material needs (the Arab slave trades), similar to what ISIS is doing nowadays.
HeavenOf all the religions that I have studied, Islam appears to be the most tangible and logical. By tangible I mean concepts like, Muslims not believing in persons directly guided by God (like the popes), all humans can make mistakes; similarly most of our laws have a beneficial risk-benefit ratio. This is the likely reason why some laws during Islam's times were different as compared to those before Islam. In Islam, the family is considered as an institute and has an extremely important and tangible role to play protecting and building-up the society. For this reason there are harsh laws prohibiting fornication and adultery to safeguard the family system. There are other reasons too, but according to scholars these are the important ones. If these were not the objectives of Islam, some scholars even argue that possibly Islam wouldn't have prohibited such things or at-least not as harshly.
These safeguarding objectives are not needed in heaven. Islamic scripture tells us that humans will get whatever they desire in heaven and if that means sexual pleasure, then they will have the opportunity for it too. Exactly, what is the meaning of "pure". Religious people base their moralities on what God allows and what He prohibits. Based on this logic, if God allows something, how can that be considered as impure. Looking at this issue from a secular objective point of view, if actions do not hurt others why are they to be considered as wrong. I believe this concept of "pure" comes from some intangible idealistic mentality through which the Christians started to regard even marital sex as a sin. They considered it as an undesirable thing; it served a purpose otherwise it was a sin to even enjoy it. To get somewhat explicit here, this is a reason the missionary position got its name, because those people thought that this was the only way to have marital relations which would not be "sinful".
An interesting comparison that can be made here, is the prohibition of alcohol in this world. If the harmful effects of alcohol are removed, would there be any need for it to remain prohibited in heaven?
Regarding the
Hadith, from what I have researched, I don't think these are Hadith, although, the Islamic concept of humans getting whatever they desire in heaven, could be construed in such a manner.
Regarding "eternal erections in heaven" read:
http://thefactsaboutislam.blogspot.com/2013/06/eternal-erections-muslim-explains-so.html
