Speaking as someone who suffers from Post Traumatic Stress (for reasons other than infidelity) I can say that the triggers a loyal spouse experiences are VERY similar to the PTSD triggers...it sends you RIGHT BACK THERE as if you are experiencing it all again. In a way, I think your body remembers the shock so vividly that your body just re-feels it. But anyway I have two suggestions.
#1--The way that I deal with or "return to the current world rather than being back there in the past" when dealing with a trigger is to find several things in the here and now that have to do with the senses: sight, smell, sound, taste, touch. LOOK at the colors of a painting in the room your in. SMELL the perfume you're wearing right now. HEAR the song that's on the stereo...or turn one on...or sing...or listen to your spouse talk...or hear a bird sing...or hear a child laugh. TASTE a sip of coffee, a juicy fruit, or something with a texture you enjoy. TOUCH your spouse...hug or kiss. Be extremely involved in NOW...right NOW...and that will help stop the trigger and bring you back to here. For me, I will even tell my Dear Hubby I'm triggering and need to sort of bring myself back here. Involving your senses in the here and now will help shorten and minimize the impact of the trigger's effect, so that it's more like a memory and less like re-experiencing it.
#2--your stepsons are not of an age to fully understand why the fact that they mentioned past "girlfriends" would have such an effect on you, and probably/possibly not of an age to have it explained to them fully. However, it is very reasonable to make a request of children like "Hey could you do me a favor? I'm a little touchy about daddy's past girlfriends because it hurts my feelings. Could we not talk about that for a while?" It's also completely reasonable to explain to children very simple concepts like "You know, moms and dads don't usually have girlfriends and boyfriends--usually they make a promise to just love each other. But you know what else? Even us grownups aren't perfect sometimes! LOL No duh, huh? Well we're doing a better job now."