Wellbutrin can cause sexual dysfunction. I would suggest you wean yourself off the antidepressant. It may take some time - but if you were not seriously depressed you don't need it!
Being upset about your sexual performance, doesn't warrant the use of a heavy anti-depressant. This drug safety is very questionable when it comes to side effects!
There is a natural way to increase stamina, erections etc. I wouldn't suggest going any further though until you get off the drugs.