"I caught my son with his hands down his pants. He's 5 yo. How do I stop him? I feel so guilty that he's doing this."
Kids aren't my field but I'm going to answer this on the basis of developmental psychology.
First of all, what do you remember from when you're five? My bet is not very much.
When we are little, we're more about doing than thinking. Kids explore and do what they enjoy. It's why they clamour for ice-cream, back scratches or a million other things. Your little boy discovered that touching his willy is pleasurable, so he does so.
You say you feel guilty and at a guess, I'm thinking you're attributing all kinds of labels that apply to adults who have mental health problems like exhibitionism. The thing is, your little son is not an adult. He's five years old, innocent and he has no idea about our taboos and shibboleths.
Most little boys and girls do play with themselves, bouncing on bicycles, rubbing up against things. Typical ages for this run from 2 to 6 years old. Ask other mums and you'll hear plenty of stories. Normally it's just a phase and they grow out of it.
Note: in some cases, the phase turns into an obsession. It's unusual and it can be rooted in boredom as well as anxiety and depression. If this happens, you should have a chat with someone who specialises in child psychology. Again, it's not a sign of moral depravity but rather a comfort seeking response because of something else going on. So it's nothing to be ashamed about but you might seek some help.
From your note I have the impression your boy isn't obsessed. However, when this is part of normal development, it's a good opportunity to explain about privacy. "Sweetie, that part of you is private. Like you go to the bathroom for pee-pee, touching that part of you is not for everyone to see."
That will help him learn about limits in a safe and natural way, and it will help you talk him out of touching himself when you've a house full of visitors.
In the meantime, try not to stress or shout from frustration. Little kids aren't complex thinkers but they are ace at emotion. So he'll see you're upset and not really understand why. He'll also feel bad about himself and for a little one that's a heartbreaking experience.
Also, avoid well-meaning others butting in with frightening tales like, "Your hands will fall off!" Scare tactics are damaging and must be avoided.
Now, about you. You say you feel guilty and that worries me.
Mums are under tremendous pressure. From what I see, you're supposed to sing while hypnobirthing, breastfeed for umpteen years, and then raise a clean-eating kid who gets straight As from pre-kindy onwards. And in your spare time you're to be a sexy, nurturing kitchen and bedroom goddess.
Please take a moment to stand back and recognise this is a load of bollocks. You're a loving mum and you're raising a boy while holding down a job. That's not easy. Take a break and realise what impossible standards are about. Be kind to yourself and enjoy watching your little boy grow up.
Have a question? During November 2016 I'm offering a free agony aunt service. Email me!