From everything I have read everywhere from anything like a reputable source, finasteride/Propecia will only cause a change or decrease in sexual desire in a small number of men while they are taking it.
Propecia leaves the body very quickly.
It cannot make your penis be less hard.
It may in some cases make you less interested in sex, notice that is not the same thing as a flopppy unit.
If you in fact have ED, PLEASE PLEASE PLEASE GO TO THE DOCTOR, as this is a major sign of SERIOUS problems in men. High blood pressure, prostate, and heart issues just to name a few.
Good luck.
Joseph