Most PD drugs reduce libido, with the exception of dopamine agonists, which can be bad in other areas. Also the whole diagnosis thing can be very stressful and depressing, and thus reduce libido further.
You can get into a Catch-22 situation where you take antidepressants to perk yourself up, but those tablets themselves add to the lack of interest.
Onc you've stabilised everything normally straightens itself out (Oops)
, and your GP can prescribe Viagra if required.