The on/off couple first confirmed their relationship in February 2011 and then enjoyed a vacation later that year.
The 19-year-old singer then told a pal the 21-year-old beauty had shown she was ''in love'' with him by agreeing to have sex for the first time during the break.
The friend told RadarOnline.com: ''Justin told me, 'I knew she was in love with me when she gave me her virginity.' He said it with no embarrassment or shyness whatsoever.''
The couple enjoyed a ''super private'' break to Palm Springs, California, to consummate their relationship and affirm their love for one another.
The source said: ''It was in the summer of 2011. They flew to Palm Springs for a couple days and had a super private place to stay.
''The purpose of the trip was 100 percent because Selena finally said 'yes' to having sex with him. They agreed because they both said they were in love.''
Earlier that year, Justin - who has been pictured in a number of strip clubs in recent months - spoke out against casual sex.
He said: ''I don't think you should have sex with anyone unless you love them.''
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