Can there be any way the doctor could know i'm sexually active by the results of my urine sample?

Possibly: If there happen to be indicators of an std that made it into your urine, or if tested after recent sex--if there is semen in your urine (carried from vagina if not practicing safe sex w/condoms), or if pregnancy test positive. Consider the risks of unprotected sex and that none of the above will show if having sex with condoms.

Not usually: Some urine samples are just tested with a dip strip,This looks for issues like sugar, protein, bladder infection, etc.It wouldn't look for STD's. If it is examined under a microscope, sperm could be seen and be evidence of sexual activity in females.If sent for STD studies it might find some ( which would be good for treatment).