Is Intimacy an Act of Worship in Islam?
Question

According to Hadith (by Muslim) there is reward for husband & wife's sexual intercourse. I heard that this reward is equivalent to the merit point of 70 Rak`ah of optional prayer. Is it true?
Praise be to Allah.Is Intimacy an Act of Worship in Islam?
 If a man has intercourse with his wife , he will be rewarded for that, because he is doing something permissible and avoiding something forbidden. This is what is stated in the Hadith of the Prophet (peace and blessings of Allah be upon him):
 It was narrated from Abu Dharr (may Allah be pleased with him) that some people from among the companions of the Prophet (peace and blessings of Allah be upon him) said to the Prophet (peace and blessings of Allah be upon him): “O Messenger of Allah, the rich people will get more reward. They pray as we pray, and they fast as we fast, but they give in charity from their excess wealth.” He said, “Has not Allah given you things with which you can give charity? Every Tasbihah (saying `Subhan Allah’ (Glory be to Allah)) is a charity. Every ‘Takbirah’ (saying `Allahu akbar (Allah is Most Great)) is a charity. Every ‘Tahmidah’ (saying `Al-hamdu-Lillah’ (praise be to Allah)) is a charity. Every Tahlilah (saying `La ilaha ill-Allah’ (there is none worthy of worship but Allah)) is a charity. Enjoining what is good is a charity. Forbidding what is evil is a charity. Having intercourse (with one’s wife) is a charity.” They said, “O Messenger of Allah, if one of us fulfils his desire, is there reward in that?” He said, “Do you not see that if he does it in a haram way he will have the burden of sin? So if he does it in a halal way, he will have a reward for that.” (Narrated by Muslim, 1674) 
Imam An-Nawawi (may Allah have mercy on him) said:
“The phrase, `Having intercourse is a charity’ – the word bud’ (translated here as `having intercourse’ ) may mean intercourse, or it may refer to the private part itself…
This indicates that permissible actions may become acts of worship, if there is a sincere intention. Intercourse may be an act of worship if the intention behind it is to fulfil the rights of one's wife, to treat her kindly as enjoined by Allah, to seek a righteous child, to keep oneself or one’s wife chaste, to prevent both partners from looking towards or thinking of haram things, and other good intentions. `O Messenger of Allah, if one of us fulfils his desire, is there reward in that?’” (Sharh Muslim, 7/92)
Is the reward of sex in Islam equivalent to 70 Sunnah prayers?
 With regard to your comment that the reward of sex in Islam is equivalent to seventy optional prayers”
Perhaps this question about the reward for having intercourse with one’s wife comes from your reading of Imam An-Nawawi’s commentary on Sahih Muslim (Sharh An-Nawawi `ala Sahih Muslim). It is a comment which we will quote below, where the Shaykh (An-Nawawi – may Allah have mercy on him) was discussing the issue of enjoining what is good and saying that it is obligatory. Then he discussed the issue of saying Tasbih and Dhikr, which he noted is Sunnah. Then he explained that the obligatory act of worship is equivalent to seventy times the Sunnah act, and said: they reached this conclusion from a Hadith… Then at the end of this discussion he referred to the phrase “Having intercourse…” 
If this is indeed the case, then you should note that the first comment has nothing to do with what follows it. The phrase “from a Hadith” means from some Hadith, but Imam An-Nawawi does not quote it. When he finishes discussing the issue of obligatory and Sunnah acts of worship, and the status of each, he then starts his discussion of the phrase “Having intercourse...” So the confusion has arisen from this juxtaposition.
This is what we think. But if you mean that the reward for obligatory acts of worship is equivalent to the reward for seventy Sunnah acts, then An-Nawawi did discuss this point and said that there was a Hadith about it, but he did not quote it.
 We know what An-Nawawi was referring to, and this is what was pointed out by Al-Hafiz ibn Hajar (may Allah have mercy on him).
 Al-Hafidh (may Allah have mercy on him) said:
 “Note:
In Ziyadat Ar-Rawdah, An-Nawawi (may Allah have mercy on him) narrated from the Imam of the Haramayn from some of the scholars that the reward for an obligatory act of worship is seventy times more than the reward for a Sunnah act. An-Nawawi said: they reached this conclusion from a Hadith.”
 The Hadith in question was quoted by Imam An-Nawawi (may Allah have mercy on him) in An-Nihayah. It is the Marfu` Hadith of Salman concerning the month of Ramadan: “Whoever seeks to draw closer to Allah during (this month) by doing some good deeds will be like one who does an obligatory action at any other time, and one who does an obligatory action (during Ramadan) will be like one who does seventy obligatory actions at any other time.”
 This is an inauthentic Hadith which was narrated by Ibn Khuzaymah (may Allah have mercy on him), but he was uncertain as to whether it was sound or not.” ((At-Talkhis Al-Habir, 3/118)
 What he meant (may Allah have mercy on him) by saying that Ibn Khuzaymah was uncertain as to whether it was sound or not may be understood more clearly by referring to Sahih Ibn Khuzaymah (3/191), where he includes it under the chapter heading, Bab Fada’il Sharh Ramadan in Sahha Al-Khabr (Chapter on the virtues of the month of Ramadan, if the report is authentic).
 And the chain of transmission of this Hadith includes `Ali ibn Zayd ibn Jad`an, who is a weak narrator.
 And Allah knows best.