1: Niall Horan was asked at a previous interview by the one and only Harry Styles, quote: "Who was the last person you had sex with?" Harry meant it playfully but I could tell the ni was hiding something.
2: Ed Sheeran wrote "Don't" allegedly about Ellie Goulding cheating on him by having sex with Niall Horan. This allegation was never confirmed.
Conclusion: I'm certain he isn't a virgin. People change.