Looking at all of your mother's precious knickknacks that you were never allowed to touch as a kid, the wallpaper that you realized has not been updated since 1996, your embarrassing baby photos your mom swore she would never show to the public -- none of this should put you in the mood.
But maybe you saw an old classmate had Neville Longbottom'ed over the years and was also back in town. Maybe you saw them take their shirt off. Maybe your dad hid his magazine in the bottom of his locked toolbox you accidentally to broke open with a brick and you happened to find it. Maybe you've been dry for 1, 2, or 5 months and you need a release because you realize not "taking matters into your own hands" until you end that dry spell is pretty dumb. So you rush to the bathroom and lock the door.
This is an obvious thought. Getting caught has been one of your biggest fear since you started to do it. The first time, all you thought about how good it felt. Then you immediately learned a huge life lesson: If it feels good there's a 99.9% chance it's wrong. So you become more and more cautious when you do it. Leave no trace, never talk about it, look no hot person you thought about in the eye again. But now you're all growed up my friend. It wasn't cute when you were 14, it's just down right disgusting and sad now in the eyes of anyone who sees you.
Mom: If mom catches you, she'll be disgusted and horrified. Her poor heart may not be able to handle that her baby would do that to themselves. You'll be disowned, you'll never be able to enjoy her home cooked meals, she won't give you any more money, she'll have other people stop giving you money, she'll tell her friends and sisters because she can't keep anything to herself, or WORSE... she'll guilt you about it. Masturbation guilt is bad enough, but mom guilt is just pure torture! Add both together.... Just stay away from heights, knives, and firearms.
Dad: If dad catches you, he'll be embarrassed as well, he might even be a bit disappointed. Then the worse will happen: he'll acknowledge it, leave in silence, and when he uses is secret agent skills to catch you when you're trying your hardest to avoid him and he'll want to talk about it: reliving the experience, telling stories about his own times he got caught holding the sausage hostage at your age (he'll probably use that phrase), maybe even giving you tips. Then after all of that, he'll ask for you to look at him, and he'll accomplish his goal -- to express complete disappointment in you, not only as a son but as a man... a man who got caught.
ETC: Maybe you brought a friend or a significant other who is very apprehensive about sex in your parents' home. Maybe it's your parents' guests who are there for reason or another. Maybe it's a sibling or other family member. They'll all think the same thing: "WHAT THE ABSOLUTE FUCK IS THIS?! You couldn't have waited until people were gone or asleep to do that shit? Just, how dare you?" Then they'll leave and you'll forever be in there debt because that will always stay with you, along with the feeling that you are a total loser for doing that there.
I assume you don't time yourself doing the Skittle diddle on the regular. When you're alone and have yourself in your hands, time just keeps on ticking and your mind just keep racing and blanking you forget about where you are or even who you are... But then you remember you're a person masturbating in their parents' home. Then you realize that someone could come to the door needing to use the bathroom, and you don't have a real reason for being in the bathroom for so long. This only increases your anxiety, but your horny heart is telling you to push on and finish what you've started.
You've finally finished. All of the fears and anxiety you had has disappeared with all the pressure you had. Your mind is finally clear again, but not for long because the fear will make a routine follow-up appointment. Luckily you tried to be as neat and practical as possible. You throw a clump of toilet paper in a toilet and hope that you can flush all of the DNA evidence down the drain. But there's just one other issue: You've watched There's Something About Mary. The only movie that has successfully added another level of fear of the five knuckle shuffle that you hadn't even considered before. You do a thorough search of the entire room and yourself. You look like a hotmess who shortened your parents' life by just existing, then shortened them further by doing all of the stupid things you do (but in your defense, that was probably before all of this). You deduce the evidence is gone and you flush. You not only flush the condemning proof of your shameful deed, you flush all of the worry, all of the fears, the whole ordeal is going away.
You wash your hands (for the love of God, please wash your hands) you can call for those darned spots to be gone all you want, but you know what you did. You dry your hands and go back into the world where you pretend nothing went on in that bathroom... at least nothing that can be pinned on you. I mean your parents don't have black lights right? If they do, that's another article.