OK, I suppose it is similar to last resort save for one important detail.
You in no way say or do anything to suggest you are giving up on your marriage.
Detaching from an ugly affair is NOT giving up on your marriage. you are protecting yourself from the person ATTACKING your marriage. This isn't in any way surrender. Consider it a tactical retreat - no more than that. And be SURE to make this clear to EVERYONE that your detachment does NOT mean you are surrendering your home or your marriage in any way. It just means the affair is unhealthy so you need to separate yourself from that. You aren't giving up on your marriage, you are removing yourself from the affair.
This is VERY important. OTHERS may mistake this as you giving up in which case the OW may pursue OM with a clear conscience. It's important that you make it clear to everyone that you WANT your marriage and your husband and family. There is a high risk of them being confused.
Regarding not wanting to hurt him :
The statistics on affair couples are dreadfully bleak. The success rate according to Phil McGraw is less than 1%. This means that if your H and OW do run away and even marry, it WON'T LAST. The OW will cheat on him or something of this sort... its textbook - affairs die eventually even if the couple thinks they have won - the AS always has the last laugh.
The damage unfortunately to your son, you, and your H is considerable. He will be miserable that yet another relationship has fallen short and he is possibly again a part time father to a second wife even.
YOU can SPARE HIM that pain by setting an example of good marital commitment. YOU can HELP HIM by PROTECTING your marriage, your family, AND HIM.
This woman is a THREAT to your marriage and YOUR HUSBAND's future as well. She is NOT good for him.
You aren't HURTING him by challenging this affair. You are protecting him. It's an act of love, not abuse.
When your son grows older and they start hanging around other boys who are a bad influence (violence, crime, etc) YOU will no doubt try to restrict him from exposing himself to that influence... its the SAME THING with your Husband. You are just trying to SAVE them a LOT OF GRIEF.
How is your letting him become a part time father, trash his marriage, and run away with this woman NOT hurting him? In my opinion he's hurting HIMSELF right now. You interfering with that to get him away from a destructive influence to his family is NOT HURTING HIM. If anything he will later thank you for taking an active role in saving your marriage.
He's under a VERY seductive influence right now and he needs your HELP.. not your indifference.
You actively challenging and putting an end to an affair is NOT HURTING HIM. YOU letting her DO this to your marriage IS hurting him...
Think about your family, think about what negative influences may be around to them (your husband and your son) and consider the OW and this affair to be one of them.. she's NOT helping your marriage at all...
If you arne't part of the solution... you're part of the problem... your H's marriage is in jeapoardy.. is the OW part of the solution?
Then get RID of her... how is that HURTING HIM?
Will he be mad that you took a stand?
Yes, he will throw a tantrum like a nasty alcoholic who just had his last bottle of vodka taken away from him. He will yell and scream and throw a nasty fit... IGNORE IT.
When your son has a fit, you don't cater to it... you just nod and wait it out.. you know he'll come around and he's working through him simply NOT GETTING WHAT HE WANTS.
Challenging your H into NOT getting what he wants is NOT hurting him... its what he needs... a grown up adult example of commitment to follow.
