Graham Sneddon, 32, who sent a series of threatening text messages to the women in England, will not be released unless the Parole Board deems that he represents no danger to the public.
Sneddon, of Falkirk, Stirlingshire, committed the offences after his friendship with one of the females turned sour.
The High Court in Edinburgh heard that one message read: "Let's see how long you have your kids after tomorrow."
He sent another message in which he detailed the objects in one of the woman's home, although the mother had no knowledge of the sex offender ever visiting her.
Sentencing Sneddon yesterday, judge Lord Pentland said he would have to serve at least 18 months in prison. He added: "Having considered all of the evidence, I am satisfied that you present a high risk to public safety.
"The punishment part of your sentence will be 18 months. I must emphasise to you that you will not be released at the end of that sentence. You will only be released from custody once the parole board consider that you no longer present a risk to the public."
Sneddon, a former McDonald's worker, had originally appeared at the town's sheriff court to admit two charges of sending text messages which contained threats of violence and a charge of struggling with the police on July 6, 2012.
He was jailed in 1998 for raping a 14-year-old boy. He also abused two girls after his 2002 release.
Sheriff John Mundy sent the case to the High Court in August last year because he was limited in the sentence he could impose.
The court heard how Sneddon and one of the mothers, who was living in the Melton Mowbray, had been in contact via the social networking website Facebook.
The victim had thought theirs was only a friendship and had asked Sneddon to stop using the phrase babe when speaking to her or saying that he loved her.
However, after she "unfriended" him on the site in May 2012, he left phone messages on her including threats to smash her partner's face in.
The woman's friend also became involved after she took the phone during one conversation and told Sneddon to calm down.
In a subsequent text, the court heard Sneddon warned her to kiss her children goodnight as it was perhaps the last time she would see them.
The friend found 237 texts from Sneddon's phone, including menacing messages and describing the various items in her home.
Sneddon's original victim was also warned: "Let's see how long you have your kids after tomorrow."
He also threatened to travel to their town, forcing them to leave their homes and go to the police.
Sneddon's home was raided and when questioned by police, threatened to stab an officer in the eye with a pen, forcing him to be restrained.
Falkirk Council took out a Sexual Offences Prevention Order against him after his arrest, which he breached twice.