Recently I was at a DNA seminar and there was a point well made. Consider the following matrix:
Source: Linens from the bed
The offense in the case is rape, the activity is intercourse, the source are the sheets and the substance found is the defendant's DNA.
The prosecution would have the jury believe that the DNA is from semen and therefore a rape was committed. The defense must challenge where that DNA came from. It could've come from a skin cell, hair, or blood. It did not necessarily come from semen. It would likely establish that the defendant was there but it does not conclusively prove that the defendant provided DNA from his semen. If there is no semen there may be reasonable doubt that intercourse let alone rape occurred.
The point is, just because DNA is present doesn't mean that the defendant did something illegal.