Will coreg (carvedilol) 12.5 mg twice a day cause erectile disfunction?

It's possible. Erectile dysfunction is considered listed as a side effect of Coreg that happens more commonly than placebo. If you notice that the side effect started shortly after you started taking it, then it may be worth having a discussion with your prescriber about how likely the ED is attributable to the med and whether an alternative med is possible.
Possibly so. Coreg (carvedilol) is a class of drugs known as "beta blockers." loss of erectile function is a possible side effect, though not that common. Often, particularly in a man as young as you, the major cause is worrying whether you'll have an erection--there's nothing sexy about worrying! still, it's a good idea to speak with your prescribing doctor to see if there's an alternative medication.