I've read this thread all in one go and it seems to me it's been side-tracked by one of the most common myths about infidelity. The myth itself has undoubtedly contributed to Bogeyface's confusion and therefore her decision to start a thread about it.
The myth of course is that the reason for all affairs is because of a person's dissatisfaction and unhappiness in the relationship.
It is well-documented now that the reason for many affairs has got nothing to do with relationship dissatisfaction and that is backed up by extensive research and testimonies from people who've had affairs. Having worked with countless couples and individuals who are trying to deal out with the fall-out from infidelity, I completely agree that this is a very common, but 'hidden' type of affair.
A poster commented somewhere on this thread that an unfaithful husband might have a different take on it to his wife. Well let me tell you, I've spoken to lots of husbands - on their own and without their partners present - who are honest enough to admit that they weren't unhappy in their relationships before their affairs and cannot use this as an excuse or a reason for their actions. When their partners ask why they didn't communicate their unhappiness, they are honest enough to reply that there wasn't any unhappiness. In lots of couples, it was just an ego boost that was regarded as a bit of fun that would never be found out.
In answer to Bogeyface's initial question, that's why so many people don't want to leave their marriages, despite having an affair. It's because there was nothing much wrong with their marriages in the first place.
To be fair, I've also talked to lots of 'third parties' (mostly OW) who also struggle with this question. They cannot understand why a man is having an affair with them if they are happy at home and want to remain in their marriages (and some men are honest about that, at least.) They tie themselves in knots trying to overlay their own beliefs onto the situation, despite what the man is telling them.
It's also easy to spot the people who are staying in their marriages because of other reasons, such as finances, children, fear of independence or failure. It would be ridiculous to suggest that none of these factors has a bearing on why people want to stay in their marriages, but the thing that stands out with people whose affair wasn't because they were unhappy is that they want to stay because they love their partners deeply and never want to hurt them again.
I don't know your circumstances Bogeyface or the reasons your husband gave for his affair or why he wants to stay in his marriage, but if he's telling you that his affair wasn't because he wanted out or was fed up with your relationship, listen up. A lot of absolute twaddle is written about affairs that doesn't bear out what some of the people having them are saying (the honest ones at least 
)