But now the time has arrived.
The overjoyed pop star make the announcement this week that the couple are expecting their first baby together.
Robbie posted an emotional message on his blog telling fans: "I've been keeping a secret from you all.
"Me and Ayda are going to be a mummy and daddy this year. We've seen scans and cried, looked at babies in commercials and teared up.
"The nursery is already planned.
"Basically I'm in love with a little person growing in mummy's belly and can't wait to be a daddy. Crying now."
He ends the entry: "Much love, much joy. Roberto, Mum and Little Bump. xxx"
Robbie, 38, and 32-year-old Ayda married during a romantic ceremony at their Beverly Hills mansion in August 2010 after four years together.
They are rarely seen on the LA social scene and prefer to stay home with their dogs, who acted as bridesmaids on their big day.
All the news as Duchess Kate's due date draws close...