Sex with daughters in law is a very pious and sacred relationship which is promoted by Quran. 
Our prophet had a beautiful daughter in law Zainab, wife of his adopted son known as Zaid Bin Mohammed. One morning he walked into her room looking for his son and ran into her while she was just waking up and was wearing revealing clothing. He liked what he saw. To make a long story short , she was in his bed within a few days.

MADINANS CONFRONT THE PROPHET

In Arab culture a daughter in law was like one’s own daughter, irrespective of whether the son was biological or adopted. The news that the prophet violated his own beloved son’s wife enraged Madinans. They confronted him.
Madinans: “Oh Prophet of Allah , how could you , of all the people an apostle of God, carry out a disgusting and despicable act of sleeping with your own daughter in Law. Your harem is bursting at seams with young and beautiful wives . On top of that you regularly take 20% cut from the crop of infidel captured women as your share of booty .
Rasulullah, your act is an incest which puts the time honored noble arab tradition of “adoption” to shame”..
Prophet: “To me the only noble and honorable act is to follow Allah’s orders . And Allah was the one who ordered me to acquire Zainab. Here is the aya”:
33.37 “We gave her ( Zaid’s wife ) in marriage to thee, so that there should not be any fault in the believers, touching wives of their adopted sons”.
Madinans: “It sounds very strange that Allah is so enthusiastic to get us married to our adopted children. Anyways , if that is what Allah wants so be it. But howcum you never had a marriage ceremony with Zainab before taking her to bed? Wouldn’t that still be called an adultery?”
Prophet: “Not at all , because I am allowed to have sex with my first cousins without marriage and Zainab is the daughter of my aunt. Here is the aya:”
33.50 (continued): O Prophet, We have made lawful for thee the daughters of thy uncles paternal and aunts paternal, thy uncles maternal and aunts maternal, who have emigrated with thee.
Madinans: “Prophet , are you sure that aya 33.50 was not brought to you by Satan like he brought to you the ‘satanic ayas’ in Mecca?”
Prophet: “I don’t get tricked by Satan any more. I am dead sure that Jibraeel brought it to me”..
Madinans: “Rasulullah ,there is another matter which we like to discuss with you. We are told that you have been sleeping with our wives after you send us on Jihad to far off places . Is that true”.
Prophet: “ Yes it is. Believing women offer themselves to me in the hope of salvation and special favours from Allah in the hereafter and Allah has allowed me to fulfill their requests by aya 33.50.
33.50 (continued) and also allowed is any woman believer, if she gives herself to the Prophet and if the Prophet desires her (to have sex) (the word is nikah which means penetrating some one).
” However, as you know I am a very busy man and cannot fulfill all requests. Besides my wives get very antsy and jealous when I am with them ; they also hate to see the long lines of muslimas outside my room daily”.
Madinans: “Actually your wives are the ones who informed us of this matter. They said that you get so drained and tired by the time you come to their rooms, you fall asleep right after prayers. Some of your younger wives looked so jealous and outraged that we could have easily taken advantage of them like Safwan Bin Muattal is rumoured to have done with resentful and angry Ayesha on the way back from Mustaleeq after you dumped her for Juvaria.. You yourself believed she did it and had sent her to her Dad’s house.
Prophet: “Allah is aware of all that. That is why he has ordered my wives to be segregated and veiled from now on. You are not allowed to come to my home without permission any more and are only allowed to talk to them from behind a curtain , if you must (prior to that madina men socialized with them freely without hijab). Here is the revelation”:
33.53: “O you who believe! do not enter the house of the Prophet unless permission is given. And when ye ask of them (the wives of the Prophet) anything, ask it of them from behind a curtain”.
And Allah has also given stern warning to my misbehaving wives.”
33.30 : “O wives of the prophet! Who ever of you commits an open indecency, the punishment shall be increased to her doubly”.
Madinans: “Can we at least marry them when you are dead and gone because poor girls look so young and unfulfilled while you are old and ready to kick the bucket”.
Prophet: “No way. Nobody should touch them even after I am gone”.
Madinans: “But you have married young widows of jihadis killed in the raids you ordered, Why cant we marry your widows then”
Prophet: “Because Allah said so”.
33.53 (contd). “And ye must not marry his wives, after him, for ever. This would be a grave offence to God”.
Madinans: “Forget about your wives . What about one night stands with our cousins and other muslimas if they offer themselves to us?”..
Prophet: “No one night stands for you. .You can sleep with them only if you marry them “…
Madinans: “Why not?. You do it ”.
Prophet: “Because one night stands is a privilege only for me , not for any one else. Here is the continuation of aya 33.50”.
33.50 (continued)” That is a special privilege for thee, not for other believers. They are only allowed their married wives and their right hands possessions (slave girls and captured women)”
Madinans: “But Prophet of Allah, does not aya 33.21 says you are our role model and aya 3.31 says we are to follow your example if we loved Allah? Then those ayas are really B.S.;because when you indulge in incest and adultery Allah endorses it right away and when we want to follow you and do the same , Allah calls it adultery and calls for flogging and stoning. Why?”
Prophet: “Allah said not to irritate your prophet and you are irritating the hell out of me”.
33.53 (continued): “It is not right for you to annoy and irritate the prophet of God.”
Madinans: “Cant we even ask any questions?”
Prophet: “No. If you ask questions you will end up getting beheaded.”
Madinans: “Where does that say in Quran?”
Prophet: “Sure it does. Ayas 5.101/102 says:
5.101/102 “Don’t ask questions. You lose faith by asking questions”.
“When you lose faith you become apostate and Allah calls for beheading of renegades even before infidels”..
Moral of the above fill in the blanks Quranic story: Your brain is like your donkey , and Islam is like a mosque. You can ride your donkey any where you want, but when you enter the mosque you have to leave the donkey at the gate.