Another flipped gender commentator here. I will say at first I was mortified libid and furious of OM's immediate interaction with my sons. I know I've gotta have some "good" posts from my earlier days in dealing with it.
My my biggest issues was that S12 is a special needs child and very impressionable. So immediately, although confused, he latched on to OM instantly and thought nothing of it to tell me of their times together. That was SO hard to consume. XW would just 'rationalize', "it's ok, they know him".
It's been a year now, and after 2 months of being D'd and XW finally seeing 'hey this is it, this is what you wanted' and that our children only see their father for a minamilist time, XW has been wistling a whole new tune. That combined with the 'honeymoon stage' is there in respect to the kids as well, and guess what, honyemoon's over and OM isn't so great to be around, and doesn't do things the way dad would. That in itself has helped me get over, not accept, OM's presence.
Sure I still get erked to see his car and expecially witness him playing in the yard, but in the end so far, they know who their father is, and who loves them as a father should. Susequently, at merely 11 and 12, they know who broke the family up, how they handle that is yet a chapter to be written.
So for what it's worth, switch the gender roles around for the situation and hopefully that helps you out.
Me 35/XW 33 S13 & S12 M: 10/17/98 OM & S: 07/08 D final 06/09/09 12/03/09 - 06/13/10 "Piercing" 06/13/10: Engaged to Re-marry 10/17/10 06/25/10: Expecting baby #3 2/14/11