You're not understanding, I don't think. If she told the SANE nurse that the last time she had consensual sex was three weeks before, and her BF's semen was in her at that very moment, it means she lied to the SANE nurse. That's number one. Number two is that such a lie SUGGESTS that Mangum was trying to attribute any injuries in her private (I use that term loosely in Mangum's case) parts she presented with as being the result of the alleged attack by the lax guys since lack of sexual activity with anybody else would bolster that as a reasonable conclusion to draw.
I understand. We just don't have enough info about that report. The lie would have influenced the SANE nurse's opinion, since the semen may have been visible to the nurse.