Might i have sex while on finasteride / propecia?

Probably: Propecia is low-dose finasteride, so is not likely to cause much alteration in libido or erectile function. So if you were able to have sex before starting propecia, you should still be able to. There is no reason not to have sex while on finasteride. In the unlikely event your erection doesn't work as well, you might go off the drug for a month or two and see if there an improvement.