After reading a few articles on the meaning of the word and this area, it seems to be a confused area.
While the word "Nikah" is translated as marriage into English, the word for marriage in the Quran and Hadith is often "tazweej" or "zawaaj". In fact, I searched through a lot of Hadiths in kitab al Nikah (the book of "marriage") in Bukhari (in Arabic of course) and found that when marriage was being discussed in the ahadith themselves, the only words used were Zawaaj. The only times I saw the word Nikah in the kitab were when they were inserted by Al Bukhari himself (for categories or commentary), in the hadiths themselves, the word nikah didn't appear.
I'll show you, here is the title of one of the ahadith I mentioned (in Arabic-I will highlight the word Nikah):
ضرب الدف في
النكاح والوليمة
Here is the main body of the hadith (without the word nikah)
دثنا مسدد حدثنا بشر بن المفضل حدثنا خالد بن ذكوان قال قالت الربيع بنت معوذ بن عفراء
جاء النبي صلى الله عليه وسلم فدخل حين بني علي فجلس على فراشي كمجلسك مني فجعلت جويريات لنا يضربن بالدف ويندبن من قتل من آبائي يوم بدر إذ قالت إحداهن وفينا نبي يعلم ما في غد فقال دعي هذه وقولي بالذي كنت تقولين
Having said that I can't find anywhere in the Quran where the word Nikah means sex either. It does seemed to have appeared from the word "istinkah" which is the root word of "yastankihaha" which can't be found in arabic dictionaries (this is the root word) and means either: "the request of sex" or "the request of marriage", this is the word used in surah 33:50. "النَّبِيُّ أَنْ يَسْتَنْكِحَهَا "
Here is an excerpt of the article I found interesting: (by Sam Shamoun)
(1) It is not at all clear whether the statement, "a believing woman if she offers herself to the Prophet, and the Prophet wishes to marry her (in arada al-nabiyyu an yastankihaha)," actually means that Muhammad could marry any woman without paying her a dowry. The text may actually be suggesting that Allah permitted Muhammad to have intercourse with any woman who gave herself to him without having to marry her! This is due to the word yastankihaha which the renowned commentary al-Qurtubi defined as:
"Yas-tan-kih" comes from the word "Yan?kah." For it is said in different forms "Nakaha" and "Istan-kaha" just as it is said "Ajab" and "Ista-jab"? It is permissible to use the word, "Istan-kaha," to mean one whom requests marriage or one who requests sexual intercourse. (Al-Qurtubi on Q. 33:50, translated from the Arabic by brother Dimitrius; Arabic source; bold and italic emphasis ours)
The following Muslim source candidly acknowledges that the word literally means to have sexual intercourse:
a. Nikah linguistically means (to unite and to bring together). The REAL LITERAL meaning of Nikah is ?to have sexual intercourse?; and the METAPHORICAL meaning of Nikah is ?the bond of marriage?. (English Translation of Sunan Ibn Majah - Compiled by Imam Muhammad Bin Yazeed Ibn Majah Al-Qazwini, From Hadith No. 1783 to 2718, Ahadith edited and referenced by Hafiz Abu Tahir Zubair 'Ali Za'i, translated by Nasiruddin al-Khattab (Canada), final review by Abu Khaliyl (USA) [Darussalam Publications and Distributors, First Edition: June 2007], Volume 3, Chapter 9. Chapters On Marriage, p. 57; capital emphasis ours)
http://www.answering-islam.org/Quran/Incoherence/mhd_marriages.htmlIn the end, it's difficult to see whether the allegations against the word Nikah are true, it certainly is the case that Zawaj is used more prominantly in the Quran as the process of marriage and that the word Nikah seems to have ended up more common today (possibly though Bukhari's use) and it is a confusing issue.
I leave it at that.