Speaking to Esquire, the Get Him To The Greek actor explained that both himself and K-Pez have "a lot of options" - which can make it difficult to be faithful to one person.
Brand told the magazine: "Some of it's the fame, but I think getting married is - you've got a whole other person that you've got to make as important as you.
"When I'm in a relationship I'm diligently monogamous and stick to the principles agreed on within that relationship. You've got to, I think."
He continued: "But it's going to be hard if I go into a monogamous relationship. I live a life where I have a lot of freedom, so if I meet someone and I go, 'Right let's be monogamous', that's a f**king change.
"But I tried it and I loved it. I really think [Katy's] a lovely beautiful person. It's just hard, isn't it? She's got a lot of options, I've got a lot of options, so you've got to really, really want it."
When asked about his womanising ways, Russell stated: "I've given over too much of my life to its pursuit. And I've probably harmed other people, by being selfish.
"And really, I'd like to be married. I'm just not transmitting that frequency of 'Soulmate! Soulmate!' I'm transmitting the frequency of 'Mmm!'"
He added: "This is the realisation of a childhood dream - my hero, the great artist Morrissey, affirming and praising my work. But while he's talking, a girl with big tits walks past in the background, and I immediately look at her.
"It's not even a decision - it's more like 'breathe', or 'kidneys separate urine from water'. It's happening on such a deep level, that stuff."
Meanwhile, Perry is said to have rekindled her romance with ex-beau John Mayer, with a source telling E! News the pair are 'causally dating' again.