Kevin Federline Says He Had Sex With Britney Spears on First Night

Kevin Federline says Britney Spears bangs on the first night ... at least in his experience.

Britney's ex-husband recalls their first sexual encounter in his upcoming memoir, "You Thought You Knew," and he says it happened the night they met.

Kevin says he was introduced to Britney at a Hollywood nightclub in 2004 and writes ... "The moment I sat down, everything else kind of just faded into the background. It was just me and her."

K-Fed says he was "just thinking about smashing" Britney and wasn't expecting to get married and have two kids together ... which we know now is what played out.

Kevin says as the night wore on, Britney invited him to join her group for a late-night swim at the Beverly Hills Hotel ... he says Britney "had a bungalow with a private pool and hot tub."

K-Fed writes ... "Then, while everyone was still swimming, Britney got out of the hot tub, grabbed my arm and led me into the bungalow. Everyone else must have got the message and eventually left. I wasn't worried about anyone else though."

When he was alone with Britney inside the bungalow, Kevin says she "turned around, slipped off her underwear and started kissing" him as she tore at his clothes with both hands.

Kevin recalls ... "We stumbled toward the bed while I struggled to kick my pants off my ankles. This. Is. Happening."

The build-up ends there, though ... K-Fed writes, "Okay. Sorry, calm down, that's as detailed as I'm going to get on that."

Still, Kevin says his first night with Britney was "purely physical" and adds ... "one night became two, then stretched into three, the whole time holed up in that bungalow."

Britney and Kevin married later that same year, had two kids and then got divorced.