I wore lacy thongs every day at that age, because I had a muddled idea that it was the "grown up" thing to do and I needed to wear "sexy" underwear to show I wasn't a child any more. I was a virgin at the time. And I'm so sure that she decided to lose her virginity to a virtual stranger, in the mud.
That said... The difficulty in rape cases like this is that it is very very hard to prove beyond a reasonable doubt that the victim did not consent. And that is the standard required for criminal conviction. If there is even a slim chance she did consent, legally speaking, a juror should not vote for conviction. It's very difficult to know how to address this, given that sex usually takes place without witnesses.
That doesn't take away from the utter wrongness and repugnance of what the lawyer said though. A woman's clothing or underwear should never be cited as though it is "evidence".