In the books, Sansa Stark has not lost her virginity to anyone as of the latest published novels. The available source says she is still a virgin by the end of A Feast for Crows , and she does not appear in A Dance with Dragons.

In other words, unlike the show, the books have not given her a sexual encounter yet.

What the books say

  • She is still in the Vale with Littlefinger and Sweetrobin at the end of the fourth book.
  • She is betrothed to Harrold Hardyng, but there is no published scene where the marriage happens.
  • So there is no canonical book answer of “she loses it to X” yet.

Show vs. books

The TV series is different: the show gives Sansa a traumatic wedding-night assault by Ramsay Bolton, but that does not happen in the books.

TL;DR

As of the published books, nobody. Sansa has not lost her virginity in the novels yet.