Been on finasteride for well over 3 years and either f*ck my girl or spank it every day.
I do recall a patch of low libido for about 3 months not long after I started finasteride. Never had problems getting hard - it was just that I didn't want to. Low libido as opposed to erectile dysfunction.
Ride it out and stop worrying about it and you'll both be fine. Only 2% of people get the sides bad, and out of that 2% the sides disappear for the majority after a few months.
Another thing I remember that helped me during my low patch was *forcing* myself to spank it. Schedule some time, download some good p**rn (whatever floats your boat), and then make yourself spank it - whether you want to or not. I think it helps - at least for the psychological reinforcement that you CAN do it.
If taking a supplement makes you feel better, then do. But your body should adjust to the finasteride.
Good luck.
Def