WASHINGTON, Feb 25 (Reuters) - President Joe Biden has joked to aides that the key to a long and lasting marriage is "good sex," according to a new book about first lady Jill Biden that casts a spotlight on their 47-year romance."

American Woman - The Transformation of the Modern First Lady, from Hillary Clinton to Jill Biden, was authored by New York Times White House correspondent Katie Rogers and comes out this week.The part about sex takes up only a few paragraphs in the 276-page book but has already generated headlines.

Rogers writes that Biden opted against running for president in 2004, a decision punctuated to aides when Jill Biden entered the room wearing a halter top with the word "NO" scrawled on her stomach.

Joe Biden, now 81, told a group of supporters that year that he had little interest in running for president. "I'd rather be at home making love to my wife while my children are asleep," he said.

The comment drew a shrug from a spokesperson at the time who said then-Senator Biden was "frankly totally in love with his wife," Rogers writes."Joe may have tamped down on his public bedroom declarations (in) winning the presidency, but he has joked to aides that 'good sex' is the key to a lasting and happy marriage, much to his wife's chagrin," according to Rogers.

The book describes the anguish Joe Biden experienced when his first wife, Neilia, died in a 1972 car crash along with their daughter Naomi.

He and Jill married in 1977, but it took five proposals from Biden to get Jill to agree."I've been as patient as I know how to be, but this has got my Irish up. Either you decide to marry me or that's it - I'm out. I'm not asking again," Biden said on the fifth try, Rogers writes.

Reporting By Steve Holland Editing by Don Durfee and Bill Berkrot