That’s not really what the quote says.  The quote seems to imply that since “it is nothing” that such a thing is permitted.  This is not the case.  However, in order to understand it, you have to understand the context of the discussion that is happening here in the Talmud.

A Ketubah (plural Ketuvot — hence the name of the tractate) is a marriage contract between a husband and wife.  One of the main points of the Ketubah is that is specifies a sum of money that the husband (or his estate) must pay to the wife if he divorces her or if he dies.  While the groom can specify any amount he wishes for the Ketubah payment, there is a minimum — 100 dinars/zuz for a non-virgin at the time of her marriage and 200 dinars/zuz for a virgin.

The Talmud here discusses various cases of a woman whose hymen was ruptured for various reasons that do not have to do with having sex and whether such a woman is classified as a virgin (and entitled to the higher minimum payment) or not.  

One such case discussed is the case where a person (unlawfully) has intercourse with a girl under three years of age.   In such a case, it is argued, she can still be classified as a virgin as it is comparable to “poking her in the eye.”  Just as poking someone in the eye does not render them a non-virgin, so too here.  

But in no case is the Talmud discussing whether such an act is permitted or forbidden – it merely discusses the potential consequences of it having happened.  Nowhere does the Talmud say that such a thing is permitted.