Reason 107 that parents suck: They are not tech savvy enough to hide their online porno trail. They leave giant cookie crumb paths such as pop-up windows (waiting to be discovered behind a CNN window) of squirting vaginas and wang enhancing drug advertisements. One minute you're about to log onto Facebook and the next you're face to face with daddy's insane fetish browsing history. God forbid you get a peak at the recent downloads section.
You thinking it's "weird" is the understatement of the week. I mean, yes, it's weird, but it's also boundary breaking, embarrassing, and for lack of a better word, straight up Blechiehfsefhfhseodhea.
I wouldn't be certain your mom doesn't know, but much more importantly, I wouldn't get caught up in that. It's not for you to know the rules of their sex life; its allowances and overall scheme. Maybe she knows and couldn't care less.
There are only two options here for you:
1. Use another computer exclusively and slowly let the trauma of knowing your dad recently watched Sorority A$$ Fiesta find its place deep in your subconscious.

2. Or, tell him to keep better tabs and control of his online history since it's a "family" computer while slowly letting the trauma of knowing your dad recently watched Sorority A$$ Fiesta find its place deep in your subconscious.