Bieber Baby??

Unlike most teenage boys, Justin Bieber is flatly denying allegations that he's had sex -- and insisting he didn't father a child with a California woman who's just filed a paternity suit against him.

In court papers, Mariah Yeater says that The Bieb propositioned her last October 25th and notes, "Based upon the timing as well as the fact there were no other possible men that I had sex with that could be the father of this baby, I believe that Justin Bieber is in fact the father of my baby."

Bieber's rep tells Radar Online "While we haven't yet seen the lawsuit, it's sad that someone would fabricate malicious, defamatory, and demonstrably false claims. We will vigorously pursue all available legal remedies to defend and protect Justin against these allegations."