"If I was marrying someone of lesser fortune who was 25 years younger, I'd be doing the same thing. Pre-nups are brilliant, because it's all sorted out."
In an interview with Vanity Fair magazine, to be published next month, the 31-year-old said she had been upset by accusations she was a "gold digger".
The Welsh actress married the 56-year-old actor in New York last month, after working out a deal which is believed to guarantee her over $1m (£700,000) for every year of marriage, should they split up.
But she told the magazine: "There was a load of rubbish written about it, but there was no arguing, no malicious feud.
"If I was marrying someone of lesser fortune who was 25 years younger, I'd be doing the same thing. Pre-nups are brilliant, because it's all sorted out."
She added: "Why should Michael be in a position where half of his fortune, which he's worked bloody hard for, lands in someone else's lap?
"It was signed and put in the bottom of a drawer, hopefully never to be seen again."
Zeta Jones also spoke of harassment by photographers after the birth of the couple's son Dylan in August.
She said her car had been "deliberately bumped" by paparazzi to try to force her out in public, leading the couple to sell the rights to their wedding to British celebrity magazine OK!
At the weekend, Michael Douglas told the BBC the couple sold the rights to OK! to control the "media circus".
"There's a phenomenon that happens when two so-called celebrities get together," he added.
"Everybody gets goofy. It gets really kind of crazy."