Hello
In addition to TJ73's point. Virgins are usually considered pure in themselves. It doesn't imply that they serve to vent our lusts. On a side note, the concept of Virgin in Tantric philosophy implies that no satisfaction from another person or object required, a Virgin is self-satisfied and perfect.
The number 70 is significant. The number 7 by 10 is related to creation. Here are some links (I have not had time to read them all), it seems to me the idea is found not only in the Quran, but in Abrahmic scriptural contexts:
The Significance of 70 Years
Meaning of Numbers in Bible: The Number Seventy (70)
The word qur`ān appears about 70 times in the Qur’an itself, assuming various meanings.
(source)
Here are some other instances of the word 70, personal impression is the number is held to high regard.
155. And Moses chose seventy of his people for Our place of meeting: when they
were seized with violent quaking, he prayed: "O my Lord! if it had been Thy will
Thou couldst have destroyed, long before, both them and me: wouldst Thou destroy
us for the deeds of the foolish ones among us? this is no more than Thy trial:
by it Thou causest whom Thou wilt to stray, and Thou leadest whom Thou wilt into
the right path. Thou art our Protector: so forgive us and give us Thy mercy; for
Thou art the best of those who forgive.
80. Whether thou ask for their forgiveness, or not, [their sin is unforgivable]:
if thou ask seventy times for their forgiveness, Allah will not forgive them:
because they have rejected Allah and His Messenger: and Allah guideth not those
who are perversely rebellious.
32. "Further, make him march in a chain, whereof the length is seventy cubits!
And Allah knows best!