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A so-called "source" tells Radar Online that JUSTIN BIEBER bragged to a friend about being SELENA GOMEZ'S "FIRST."  Justin supposedly said, quote, "I knew she was in love with me when she gave me her virginity."

 

The friend says it happened in the summer of 2011, when Justin and Selena made plans to go to Palm Springs SPECIFICALLY to do the nasty.

 

The friend adds, quote, "The purpose of the trip was 100% because Selena finally said 'yes' to having sex with him.  They agreed [to do it] because they both said they were in love.