On The Early Show Wednesday, Levi Johnston also said things are improving in his relationship with Bristol, as he seeks more time with their son, Tripp.
Bristol's pregnancy became a major issue early in the presidential campaign. Her mother, Alaska Gov. Sarah Palin, was Sen. John McCain's running mate. Bristol, 18, and Levi, 19, weren't married and have since broken up. Tripp was born Dec. 27.
Bristol was in New York Wednesday to help promote National Teen Pregnancy Awareness Day. She's been named a teen ambassador for The Candie's Foundation, which is sponsoring a town hall about teen pregnancy prevention.
She went on national TV to say she believes abstinence is the best and safest choice for teens.
Johnston told Early Show co-anchor Maggie Rogriguez Wednesday he agrees. "Abstinence is a great idea," he said, "but I also think you need to enforce, you know, condoms and birth control and other things like that to have safe sex. I don't just think telling young kids, you can't have sex, it's not going to work. It's not realistic.
" ... It's a great idea and a great message she's trying to send out to the world and all the young kids. It's not easy raising a baby. But I do think there's more things to it than just not having sex."
Asked if he was using condoms when Tripp was conceived, Johnston said he was, "but there's a few times we didn't, and that's what happened. We had a kid."