I'm 23 and it used to happen to me every so often.  Of course...
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				 healthy libido *with her*
			
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 ...it only happened with one particular girl.  I have no idea why.  She was cute, she was fun to fuck, she always wanted to fuck, I just couldn't make it happen with her.  I couldn't stay hard with this girl for the life of me. 
Solution --
New girl.  Haven't had the problem since then.  I dunno if this applies to you, but I thought I'd share.
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				That's when I went on the pill and we didn't have to worry about condoms.
			
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 Condoms suck.