I know Bell doesn’t write with much depth, but I think there is the opportunity for it with Queric.
Maybe Eric’s problem was never physical and was caused by his wife’s betrayals. Yes I know Eric has done plenty wrong in his life and he forgives almost everyone of everything. Eric rarely forgives his wives though. The only time I can remember him forgiving and staying with a wife was when Ridge and Quinn had their fling.
Eric could have a complete mental block with Quinn that is preventing him from being intimate with her. That would explain why he could get aroused with Donna but couldn’t perform with Quinn. He obviously loves her and wants her, he could have had Donna in that moment but he left to find Quinn. Eric’s problem could be subconscious and he doesn’t even realize it.