The 55-year-old star admires his character, St Clair Bayfield, in the film "Florence Foster Jenkins", who had a mistress, because he thinks casual affairs strengthen marriages, reported Female First.
"I always admire the French and the Italians who are very devoted to their marriages.
"They take them extremely seriously, but it is understood that there might be other visitors at 5 o'clock in the afternoon. You just never boast about. They never say anything, but that's what keeps marriages together," Grant said.
"I can see the lovely aspect if you marry exactly the right person - your best friend and it's cozy and it's lovely. But, people make so many mistakes.
"Do I think human beings are meant to be in 40-year-long monogamous, faithful, relationships? No, No, No. Whoever said they were? Only the bible or something. No one ever said that was a good idea. I think there's something unromantic about marriage. You're closing yourself off," Grant said.