Minesh Parbat was driving at 60mph when he crashed in West Sussex in March last year.
His girlfriend and mother-of-two, Lisa Watling, was thrown from the car and the 28-year-old died later in hospital from her injuries.
In the aftermath of the collision, Parbat was discovered by members of the public with his trousers and underpants around his ankles.
Watling was found seriously injured in just a T-shirt and bra.
Jailing him at Lewes Crown Court in south east England, Judge Peter Griffiths said: "I'm satisfied that the cause of the accident in which Lisa died was your decision to continue to drive your motor vehicle at a speed approaching but not exceeding 70mph while engaged in some form of sexual activity with your trousers partially down.
"I make it clear that the blame for this tragic accident is entirely yours."
During the trial, Parbat denied anything sexual had taken place as he drove, adding that he had instead struggled to get Watling off him before crashing.
Parbat later added: "It's got sexual tones, but it wasn't sexual because nothing happened."
He suffered facial injuries in the crash and gave a positive blood test which showed 102 milligrams of alcohol per 100 millilitres of blood, the drink-drive limit in Britain being 80 milligrams.
In a statement, Watling's family said: "Our lives have been ripped apart, leaving a wound that will never fully heal. However, the two real victims are Lisa's children.
"They will never have that soft comforting voice they know and love whisper goodnight, safe in the knowledge that when they wake up in the morning she'll be there to greet them.
"As a family we feel relief that some kind of justice has been seen. It feels like a very small step towards gaining a degree of closure."
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