There is an expression "米已成炊" (rice is cooked) It describes "既成事實" (a realized fact)
When raw rice is cooked, it cannot be changed back to raw again. Old Chinese thinking considered marriage is permanent, must be treated with caution. If a man and a woman get married, it is like raw rice boiled to be cooked rice-- It is permanent, cannot be undone. The only option is to eat it, meaning accept it
This quote is not the same as the original Chinese one, just borrowed the concept of "marriage is like cooked rice"
In the past, if a man and a woman had sex, they had no choice but to get marry. "米已成炊" can also describe this fact.
The quote from your link said: "一個美好的婚姻是用冷水準備並輕輕煮沸的米飯" (a beautiful marriage is like rice cooked from cold water and slowly boiled) implies "to ensure the marriage is a happy one, you should take time to prepare and not rush to commit."
The Cantonese version is "生米煮成熟飯" (raw rice cooked to become cooked-rice)