The truth behind Kim Cattrall‘s bust-up with young boy toy!

Sex and the City star Kim Cattrall’s five-year romance with much-younger boyfriend finally crumbled because he wanted a baby and she said no way!

The 52-year-old actress and her 23-years-younger live-in lover Alan Wyse went their separate ways recently after Kim rejected his proposal to adopt a child together, according to sources.

"Kim had no trouble keeping up with a much younger man – but when it came to starting a family, the age difference mattered big-time!" an insider told The ENQUIRER.

"She was happy and content with Alan as long as he played by her rules.

"But earlier this spring, Alan started grumbling about how his life felt incomplete and that he wanted a child. Kim soured on Alan as soon as he started talking babies.

"She’s way past the point of wanting to deal with the hassles of dirty diapers and being tied down for the next 18 years of her life.

"They hit an impasse," the insider divulged, "and with neither willing to budge, Kim and Alan decided it was better to part ways now rather than drag things out."