The 28-year-old also opens up to ET about the reasons behind his virginity.
In his new book, The First Time: Finding Myself and Looking for Love on Reality TV, the former football player opens up about his virginity and how he spent some of his adolescence and early 20s questioning whether he was gay. In an interview with ET's Lauren Zima, Underwood credited The Bachelor for helping him realize he was straight.
"[The show taught me] that I'm straight and I'm very, very attracted to Cassie [Randolph] and women -- but it would have been OK if it would have been the other way too," he admitted. "I think that's the biggest message I have for people."
"If anybody takes anything from this or is going through this, if I help one young man or one young woman go through something that they're struggling with -- to let them know that they're not alone -- then I consider the book a huge success," Underwood added.
Underwood first publicly revealed his virginity during his appearance on Becca Kufrin's 2018 season of The Bachelorette. Almost immediately after that episode aired, Underwood, 25 at the time, remembered seeing headlines saying he was gay, closeted, or "doing this for attention."
"Even now, I still battle gay rumors when I'm with Cassie, but that's how it was for me as a young kid in grade school and high school," he said. "I can deal with them now. ...People, sometimes when they don't understand, they have to get from Point A to Point B somehow, and that's a line that they draw. That's just what they do to make sense of things in their mind."
That understanding, Underwood said, comes from him previously being "stuck in a hyper masculine culture" with football. He also grew up in a conservative, faith-based family, and "lived within boxes" that said he couldn't play football and be gay, or "walk with God" and be gay. The frustrating part for Underwood was not being able to really explore the answer to those questions about his sexuality. "The struggle for me was like, 'How do I talk about this with anybody?' I didn't," he shared.
"There was a rumor going around my high school that I was gay because I broke up with one of my girlfriends at the time. It even got to my mom, where my mom was pulled me aside and said, 'You know what? We'd still love you if you were gay.' I was like, 'I appreciate that, but I gotta figure this out,'" Underwood remembered. "It was a little awkward."
The fact that Underwood himself had questioned his sexuality wasn't addressed throughout his appearances on The Bachelorette, Bachelor in Paradise and The Bachelor. The reason behind his virginity also wasn't clearly stated -- but Underwood told ET that's partly because he was still "insecure" about how to handle it.
"I really didn’t know if there was an answer that I should be saying or shouldn't be saying," he explained. "'Why can't I just say it's because I'm a Christian, because I believe you should wait till marriage?' Because that's not 100 percent accurate. That's not the 100 percent sole reason why I'm a virgin."
It was a mixture of reasons for Underwood, who details those in his book. "Between my parents' divorce in college sort of messing me up, between being bullied in grade school and high school and literally Googling, 'Am I gay? Why don't I want to have sex?' and then internalizing it all and sort of moving forward with football. I think it's a mixture of all those," he said.
"Now that I can look back, that's what made me, me," Underwood continued. "There is no one thing that is on your road that changes your life. There is very rarely that one instinct, and that's the case with this virginity. It's not just one thing. I wish I could say it was all God, because I know that's what he wants for us, but it's not. That's not the case."
Amid questions about his sexuality, Underwood said he "never acted on anything."
"There was questions there, and I either internalized it or moved forward with football. And going on The Bachelor was a way for me to not be able to run away from a relationship, not to be scared, and to open up," he shared. "I'm so grateful for the franchise for helping me grow, but I continue to have moments of self-reflection to realize, 'Hey, maybe this is why I am the way I am.'"
"I think that's so cool and so therapeutic about writing this book in the first place. It was challenging at the time to really push myself and really ask myself some of the difficult and hard questions -- to really figure out who I am as a person," Underwood continued.
The 28-year-old said his book title, The First Time, is a nod to taking back a narrative that he let "get out of control."
"It's taking ownership of, 'Hey, yes, that's me. That's who I am, but I'm a lot more than just that, and I want you to hear it from my side,'" he offered. Underwood's life experiences provided challenges and forced him to question himself, but he wants readers to know "it's OK not to have all the answers and not have it all figured out."
As for when fans will find out about when Underwood finally had -- or will have -- his "first time," he joked it could be if and when Randolph ever announces a future pregnancy. "You sort of get a two-for-one when we decide to have kids," he quipped.
For now, "Cass and I, for our relationship, have decided we don't want to share that." "We sort of just laugh and smile, and move on past it," Underwood added.
The First Time: Finding Myself and Looking for Love on Reality TV is now available where books are sold.