Hawke has two children with ex-wife Thurman and two with current wife Ryan Shawhughes, whom he married in 2008.
"My relationship with my present wife is thrilling to me and I'm committed to it," he said. "But neither she nor I know what shape the future will come in. Sexual fidelity can't be the whole thing you hang your relationship on. If you really love somebody you want them to grow, but you don't get to define how that happens. They do."
"People have such a childish view of monogamy and fidelity. 'He's cheated so he's bad, she's cheated so she's bad,' as opposed to a recognition that our species is not monogamous," he added. "To act all indignant, that your world has been rocked because your lover wasn't faithful to you, is a little bit like acting rocked that your hair went grey."
Hawke said he and Thurman "tried to be as optimistic as we could be" about their marriage, which lasted six years. The two met on the set of their 1997 film "Gattaca" and divorced in 2004.
" 'Gattaca' was a great moment in my life and a great moment in Uma Thurman's life, and we fell madly in love and wanted to do all that," Hawke said.