There is no such thing as "live DNA".
What exactly do you mean in terms of molecular analysis that was done. DNA can exist for years and be identified. How is this "live DNA" evaluation done?
You tell me, how does semen get into panties?
Did they do a semen test or DNA test or both?
Look up "motile semen."
Did they do a semen test or DNA test or both?
Uh, semen contains DNA.