I personally would not have said anything to the other mother and simply made an excuse for my son. I'm sorry if that doesn't hold true to some but this other mother is not her friend, its her BOSS. It'd be different if she'd known this mother somewhere else, were best friends, etc but she's in a position of authority and could cost her her job.
Here's what Boss mom is thinking when she's telling her this: My son COULD NOT BE Gay cause I asked him and he is in sports and blah blah (insert what ever stigma you want here it all fits) and then to blatantly ASK her already confused kid about what happened...well...not a good scene at all.
I would have advised my kid to mention it at some point if they were GOOD friends. Not to mention it to others (kids are SOOO cruel at this age) and then to leave it alone.
The touching thing...omg...when is it that parental units ARENT going to automatically jump on the PEDOPHILE bandwagon. Stop with the whole "that could be a criminal act" BS. So annoying.
Anyway, now you gotta do damage control. Don't mention it again. Tell your son not to mention it again but that you are open to have any conversation about it at any time. And hope that your son doesn't suffer the ramifications of you not being able to be your sons confidant and nothing more.
Also, this was a situation between your son and you. Not between your boss and you. There are times in your life when someone asks you a question that the appropriate answer is simply "none of your business". This would have been one of them. (Water under the bridge I know)
I would undertand you needing to tell her if the boys were doing something lacivious. But I would have never gone to another parent with "holding hands".
I wish you good luck and am sending good thoughts your way.