I can totally understand how that would disturb/freak you out. I'd be wiggin' too. But like a lot of other people have said, she is a grown ass woman. You don't know what her relationship with your stepdad is like behind closed doors. They could be swingers, he could enjoy knowing she talks to other people, she could have caught him cheating, he could be a fucking asshole, or she really is a whore. Either way, it's not really any of your business where she's throwing her pussy, unless it's her fucking your significant other or something crazy like that. lol Just stop getting on her page and looking at it. That kind of shit will scar you for life, and she'd probably be SO fucking embarrassed if she knew that you knew.
My mother once told me that there are things in my life that she will never need to know, and as such, there are in her's as well. So I figured that parents are people too, but they're people that you don't really want to know anything about because then it makes it weird and hard to see them the same. But ya, you don't think Pornhub got all those step mom videos from Brazzers.... Did you ? lol
Eh, it happens. I consider my mom to be a pretty decent, open-minded sort, but... Once when I was a teenager my mom had a few too many glasses of wine and began stroking my friend's cheek, longingly, telling him things like "Ohhh, Bobby...you are so handsome, just like your father!" (??)
I'm not gonna ever say what I did drunk one time but I can tell yas it wasn't pretty. No, not my mom you perverted fucks!