Morris - I agree with your point that "And I don't think it is an act of evil or of moral repugnancy to 'take a father from his kids' because dads are still dads wherever they live, and adult men make their own decisions."
I agree that men make their own decisions - and any notion that "the nasty OW made him do it" is bollocks. This infantilises men, who make their own choices.
I do think however, we have a responsibility to be kind and decent to our fellow human beings. If a man I cared about was talking about how unhappy he was at home, the last thing I'd do would be to make him even more confused by taking him to bed. All of us are open to temptation and there's nothing more flattering than an ego boost from someone of the opposite sex.
A lot of couples go through rough patches and difficult times - injecting a new love interest into the mix is however more likely to exacerbate the end of a relationship that could have been saved. To me, this becomes an unfair fight as well.
I've been in this position myself too - over the years, for some reason I've had umpteen men tell me how unhappy they are at home. If circumstances had been different, some of these have been men I'd have been attracted to as well. But my response has always been the same - suggesting counselling, trying to get them to see it from their wife's point of view etc. Giving the green light in these circumstances would be the knock-out punch when the couple are on the ropes - hardly fair and not the actions of a decent person.
And Mummee - I profoundly disagree with your assertion that "but if a man is happily married he is not going to be unfaithful to his wife" - I'm afraid that is nonsense. There are sadly a whole swathe of people who are actually pretty happily married, but because they are feeling a bit down or even severely depressed about other issues in their lives, will accept an opportunity to be unfaithful, thinking that this will jolt them out of their depression or mid-life crisis. It's a form of escapism and is because of a weakness in them, not their marriages.
Many women will tell you on here that prior to their husbands having an affair, they were having regular and enjoyable sex - and their husbands were telling them how happy they were in the marriage. This is why they are mystified when an affair is uncovered. Unfortunately, sometimes it is just opportunity - and sometimes opportunity striking at a time when the married partner is having some other, non-marriage crisis - that provides the conditions for the affair.
So in summary, attached partners have free will to say "no" - and if they don't, it's their fault. But decent human beings support marriages in trouble, not exacerbate their downfall.