That's an interesting question. You would think it might - it is after all a muscle relaxant - not exactly something you'd want to have happen during sex but there is little evidence that it does. Reduced libido is a symptom that occurred rarely enough in Flexeril studies (<1%) that investigators couldn't determine whether Flexeril caused it or not. Oddly enough one study suggested it could cause either increased or decreased libido.
This is a grey area; if Flexeril does cause erectile problems it would happen very rarely. I suggest you talk to your doctor about this and see what he/she says.