Since it's not defined there's no way to know for sure but:
We know that Gendry
- Had Royal Blood in him
- Was a Virgin
- His Royal Blood was used in offering to the "Lord of Light" asking for 3 names to be killed.
3a. "The usurper" Robb Stark (dead)
3b. "The usurper" Balon Greyjoy (dead)
3c. "The usurper" Joffrey Baratheon (dead)
- Gendry Was supposed to be sacrificed to the "Lord of Light" to make Stannis Baratheon King before he was set free
We see that 3 of the 3 names that were asked to be killed have died but they haven't made their "great sacrifice" for Stannis to become King.
Similarly we see that Jon Snow also has Royal Blood, which could make him a contender for her ritual. She seems to have a little liking for the "young wolf" and may not want to see him dead.
Of course it could just as easily be argued that it is purely to gauge the likelihood of him being swayed to break his vows. As a member of the "Night's Watch" Jon is not supposed to sleep with women (actually it only says take no wife nor father any children, I'm pretty sure there's a little bit of wiggle room in there). If Jon has broken his vow not to sleep with women he might be more willing to break his vow and leave the Night's Watch to help King Stannis.
This theory, however, falls short when you realize that he could have lost his virginity before joining as most of the men there probably have.