It was curiosity, plain and simple. It's normal. We all wonder "what could've been" and "what if..." when we find our relationships somewhat the same ol' same ol'. We tend to let our minds wander and sometimes they wander back into the past where they really shouldn't go. I'd say to take up a hobby or something to keep your mind on the family in front of you. Put a little more time into your marriage instead of trying to find free time to think of an ex. Your husband would probably appreciate it and that might help you feel needed and wanted by the man you're committed to.
Look into your marriage... is there something you can change about yourself? Do you dress nice or just throw on any old thing that's (sometimes) clean? Do you treat him like the father of your children or just the guy that lives in the same house? Do you show him you love him with little things like putting toothpaste on his toothbrush or leaving him little notes, stuff like that? Try it and get more in tune to him and your kids and you'll find yourself forgetting about that ex. There's a reason he's your ex, right? Hope this helps.

