"Every day they teach me different things," the pop star, 39, tells Parade of his boys, born via surrogate in August 2008. "The love is there. When you have a 2-year-old saying every other hour, 'Papi, te amo. Papi, I love you,' it can't get better."
Certainly, there's never a dull moment. "A lot of people say, 'Wow, you're a single father of twin boys, that's crazy!' " he says. "Two toddlers can get hectic, but I wouldn't change it for anything."
Asked what fatherhood has taught him, Martin replies: "Heavy lessons about simplicity and unconditional love. I don't want to sound cliché, another father talking about unconditional love, but man, that's the way it is. You hear it all the time, but when you feel this other branch of love getting stronger and stronger every day, it's just amazing."
He also reveals that the twins are already following in their famous father's footsteps. "The other day I took them to a toy store and they went straight to this tiny piano," he says. "And I'm like, 'Look at the robot!' But no, it was the piano. Both of them. So it's very beautiful."
Martin, who came out last spring, also talks about being a voice in the gay community – and says it's a natural progression for him.
"It's something I feel really good about doing," he says. "I have been an activist for human rights for many years. Today, I'm in touch with who I am and I have the opportunity to be in front of a camera and talk to millions of people."