What are 72 virgins promised to willing to die for a cause..?

Answer 1

Some interpretations of Islam teach that if a person is martyred as a result of doing God's work, they will be rewarded in Heaven with 72 virgin wives.
It is reward promised by Almighty Allah in the holy Qur'an, and the holy Prophet s.a.w. in his sayings. But the promise is not restricted to mytered ones. It is general promise done to each man who goest to Jannat (paradise). These are the wives of that person. The number 72 is idiomatic and figurative, not exact figure. It means 'many'. It is extremely unfortunate that the satanic minds are using it to persuade innocent youths to explode themselves killing innocent human beings. It is utterly Un-Islamic and condemnable act.

Answer 2

This refers to a myth that when a Muslim martyr dies, he promised 72 virgins.

The Quran does not promise martyrs 72 virgins in heaven. It does mention companions, houri, to all people—martyr or not—in heaven, but no number is specified. The source for the 72 virgins is a hadith (a saying that's not part of the Qur'an) in Sunan al-Tirmidhi by Imam Tirmidhi.