I'd like to add some thoughts to this discussion.
Obviously, there are "differences" between an MLC affair and an affair that does not involve one or both affair partners being in MLC.
I strongly believe, however, that getting romantically/physically involved with any person who is married, even if they're separated, is a dangerous, dishonest thing to do and I have reservations about the character of anyone who would make the decision to do so.
Even if the story the married person is telling the affair partner is the most heart rending, horrible thing imaginable, and even if it's all true, I believe people who respect the rights of others would chose not to become an affair partner until the other person had honorably and with honesty ended their marriage.
All affairs involve lying, manipulation (usually of the clueless spouse,) betrayal of vows, excruciating pain, and potentially deep damage to children, if there are any. Even affairs that result in reconciliation or never trigger a separation of the married couple do major, long-lasting damage, especially in the area of trust.
Can affairs be "good" for some marriages in that issues that needed addressing are finally brought out into the open? Yes, that's true, but I refuse to believe that an affair is the only way such issues could have been addressed.
While I agree that my H's affair has forced me to do some very serious soul-searching and has afforded me the opportunity to deeply look at what were the dysfunctional ways of relating to each other that contributed to my H's crisis, I cannot say that the affair was the only way this work could have been done.
Perhaps with more time I will have a different opinion on this. Perhaps I will come to see that my H's affair was the only way I was going to "wake up" to the things I was doing, or not doing, that contributed to my H's decision to leave me and live with someone else. Perhaps.
But at this point in my journey, I still think the price has been too high--for me, for my H, for my D, for the OW's H (who she divorced to be with my H) and, I strongly suspect, for the OW's adult children.
While I think most divorces could and should be avoided, I realize that some are justified. Affairs, however, are dishonest, destructive, cruel methods of extricating oneself from an impossible marriage. I don't believe anyone should be a part of an affair, no matter how "dead" or dysfunctional the marriage might be, nor no matter how much in love they are with the married affair partner.
My 2 cents.
TMHP
M 40 yrs.
BD 1/11
Began living with OW 1/11
Divorce final 8/13
Ex married OW 6/15
God, grant me the serenity to accept the people I cannot change; the courage to change the one I can; and the wisdom to know it's me.