I think it's natural to feel jealousy towards someone who you perceive has taken something away from you. Even when the logical part of you knows that your DH is the one responsible, there is still the feeling that if she wasn't around to tempt him, it wouldn't have happened.
There's comfort in that, because to look at it any other way necessitates a whole new perspective on your DH. As long as the OW is deemed an evil temptress, the problem in the marriage is confined to "the affair" - as if everything else in the marriage was and is fine, so as long as the OW is removed from the equation everything can get back to normal once the initial hurt has died down.
It is a perspective that allows hope for the future. Sometimes, it's also true that everything in the marriage was fine - it's a misconception that the relationship has to be struggling for someone to get tempted. Quite often, the source of discontent is external to the marriage, e.g. fed up with the slog of a dead-end job, or worn down by children, then someone comes along who makes you feel alive and desirable and so offers a temporary respite.
Unfortunately, even in cases where this happens it is a mistake to allow any natural antipathy towards the OW to let the man off the hook (or vice versa if it's the DW who's had the affair). Unless he addresses the factors that made him choose to go with it (and it always is a choice, even if a very un-self-aware one), he will always be vulnerable to another.
Whether the OW is naive young thing, scheming temptress, or just an ordinary woman who's made a poor judgement is actually rather irrelevant if you really want to get over the affair either by rebuilding a marriage or by leaving.
All that said, when the OW is someone who is fairly well known to the betrayed spouse, I think it's very different.