Here's my take on it; it happens, ok? We are all human. And sometimes, much to our chagrin, it happens at the most inopportune times, like right in the middle of a session, especially when certain, ahem, activities are taking place.
I always feel bad for gents when it happens to them, especially during my pm sessions, because I think they are really embarrassed and I don't want them to feel bad.
Just make a quick little polite gesture (pardon me, etc) and move on. It's not that big of a deal, and it happens to us all.