I've seen some brilliant discussions about this on other threads and there are posters who explain this very well, but I'll give my own thoughts for what they are worth.
I think we live in a world of conflicting cultures. It's widely regarded as a huge positive to have friendships with- in this case- men and as you'll have seen on Mumsnet if you've been around a while, the 'group think' is always that it's entirely possible to make friends within the sexual group(s) you're attracted to, without it leading to sex. Even if sexual attraction is there.
So far, so right.
But then when we step out of that optimistic view of human nature, we notice that despite all that, affairs with people who started out as friends or colleagues are common.
So the culture is to be positive about friendships and negative about affairs.
I'm not suggesting this is what happened to the OP, but I can envisage a situation where this boss-employee relationship started very positively and the OP was overjoyed she had a great boss who she got on with so well. Likewise he might have been pleased to have such a good member of staff who seemed to 'get' him. Because the culture is that this was only a positive and good thing neither might have put in any boundaries at all to this developing into something else entirely.
I don't think it's that people fail to register this is dangerous territory, although there's a pay-off in not doing so until it's too late, it's that people trust themselves and their own willpower way too much. So before that first overnight conference, or drink after work 'to discuss a sensitive business issue' I can imagine these two cognitively recognising the danger, but promising themselves they wouldn't let things get out of hand.
That final decision probably doesn't have the significance you suggest. By then the addiction's so strong and the boundaries so weak or non-existent, that in the place of the sort of rational risk-assessments you describe about it being an aggressive act with horrendous consequences, some really banal rationalisations take their place.
No-one need ever find out. This won't hurt anyone. It's just a bit of fun. If we don't fall in love it will be okay. What the eye can't see. This isn't dangerous at all because I'm happy at home and will never want to rock the boat.
Even If I was going through a bad patch in my marriage, I wouldn't risk it because I'd be more likely to fall in love and want to leave. But we're not, so this is never going to threaten my marriage
I'm not saying BTW that affairs are never intentionally aggressive acts, or that aggression and punishment aren't sometimes there. But I don't think that's a primary motivating factor in these 'bit of fun' affairs.
I can see that some men and women resent having to go to work and having a life chockfull of responsibilities and if the other partner appears to have an easier life or can cope better with life's challenges, maybe there's something about weaker characters giving themselves a secret 'treat' that they regard as all theirs and which gives them a prop to cope with their resentment, but usually this is seen again as a good thing because instead of snapping at the kids about treating the house as a hotel or snarling at a partner that they've got the life of Riley, in the initial stages these resentful characters are probably easier to live with.
I don't think that lasts though. Usually the stress, the lack of emotional presence and occasionally guilt kicks in and the arguments at home get worse, not better. At least for a non-compartmentaliser.
Finally, I don't think we're talking about people who secretly harbour hate or massive aggression towards their partners and children, much beyond normal 'is this my life now?' territory that I reckon everyone feels some times, however fundamentally content they are with their partners. But I think we are talking about ordinary people who think they are stronger-willed than is often possible, once an addiction's taken hold. People who aren't self-aware at all who can recognise the signs of addiction in themselves- and are then too selfish to back away.