A police officer said: "He told doctors he cut his privates on his own."
The unnamed couple had been going out together for about four years.
Kotwali police chief Anil Upadhya said: "A case of attempted murder is being registered against a 20-year-old girl who is a neighbour of the victim.
"They loved each other but the boy's marriage was fixed with some other girl as both their parents had some caste issue.
"It is due to this that the girl executed the act, revealed our primary investigation.
"The girl was in a relationship with the youth for three to four years
"She was unhappy as the boy's marriage was arranged with another girl of his own caste from the same village.
"The girl invited the victim to her residence and cut off his private part using a reaping hook under the pretext of having a physical relation."
She has been arrested and faces an attempted murder.
The sharp curved sickle, used for cutting crops, has been recovered - along with the severed penis.
Local sources say doctors are unlikely to be able to reattach it.
The man, who had returned home just 10 days earlier after working in Mumbai, is recovering in Sanjay Gandhi Hospital in Rewa.
It is the second gory incident of its kind in the area in less than a year.
Last February, a woman, 30, cut off her 22-year-old brother-in-law's manhood after she said he tried to force himself on her.
He later hanged himself from a tree branch.
