Ryan Reynolds has joked that he knew Blake Lively was ''the one'' after they had sex.

The 'Deadpool' star - who has 23-month-old daughter James and a four-week-old baby girl, who is to be named, with his wife - came up with the witty response that it wasn't until he bedded the 29-year-old beauty that he realised he was smitten with her.

Asked when he knew Blake was the right girl for him, he quipped: ''Probably after the sex!''

The 40-year-old heartthrob asked Blake to dance in an empty restaurant in Tribeca, New York on their first date and hinted that they had sexual relations immediately after.

He told the new issue of GQ magazine: ''It was just one of those moments where halfway through the dance, it was like, 'Oh, I think I just crossed a line'. And then I walked her home. And, uh, you know, I don't really need to go into what happened after that.''

The 'Age of Adeline' star threw a 40th birthday bash for her hunky husband last month at one of the first places they went on a date together, where she says they ''fell in love''.

At the time, Blake wrote on Instagram: ''We fell in love at your restaurant in Boston. We stay in love at your restaurant in NY. Thank you for the BEST food and memories! You are our FAVORITE!! Another happy birthday with @vancityreynolds . Hey, Reynolds, I like you a whle lot. Just sayin'... (sic)''

Ryan also took to the social media site to praise Blake for hosting the event and to thank their kids for not ''ruining everything''.

Adding a caption to a picture of the couple with comical birthday masks on, he wrote: ''Best. Birthday. Everrrrr. Courtesy of my wife, buddy, and international spy, @blakelively. With huge thanks to my favorite restaurant in NYC, @_o_ya_ - and a special shout out to our children for not ruining everything like they planned. (sic)''

Blake and Ryan met while shooting 2011 movie 'Green Lantern' and wed a year later.