1 I have to do it now

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.

 

2 What if I get caught

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. 

 

3 Who might catch me

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.