Bobby, 51, told Heat that his son Niall has never had a proper girlfriend before.
"He never really brought girls home," Bobby told the magazine.
"He had some girls that were friends and they would all sit on the wall outside together, but that was all."
Niall's dad added: "He's never had a real girlfriend... he went to an all-boys' school so it was hard to meet girls, but that's certainly changed now.
"I don't believe he's a virgin. I couldn't say, and I wasn't in the house all the time, so I don't really know what went on when I wasn't there."
Horan's bandmate Harry Styles quipped last year that he would probably be a virgin if he was not a member of One Direction.