"Rule of Relationships"; never tell a person who is in a romantic relationship anything important to you, unless you fully understand they will likely tell their partner before you do.
It sucks, I know. :\
I've completely refrained from telling entire circles of people because I -know- that all it takes is one couple to ruin the entire thing.
I told my best friend David a while back, and he told his girlfriend without me so much as hinting that it'd be okay for him to do so, but I expected him to tell her anyway, so it wasn't too difficult for me to wrap my head around.
People suck, yeah. Unfortunately the people you trust aren't always going to be respectful of your wishes, no matter how much they say they will be. I know I've lost a relationship in the past simply because I didn't want to tell my partner every little secret of my long-established friends, but I know that a good deal of them wouldn't make such a sacrifice for me.
I'm really sorry you have to go through this, but maybe you can still find a way to talk to Tom about it without letting anyone know that people have been blabbing when they shouldn't be?
Good luck. :]