I peeked with Zach in my main PT (Alexis path). I wouldn't do that IRL, but it's a porn game so we can have some fun, and a peek is not that bad. But Elena and Laya forgave me since I have good relationship with them. Though I chose to be mad with Zach after his slip-up. Don't remember if Alexis said something about it, but since Layla is ok it must not have been much
According to the walkthrough it ends the Layla romance("just friends with Layla" flag activated), the same flag that turns on if you chose Alexis over Layla... so long term they remain friendly, but romance chances are over.
Goes without saying that by ending the romance with Layla it ends the Elena path too.
Val may still be possible despite taking a -4 hit (Layla also takes -4 and Elena -10 but relationship chances with them are over anyways).
No effect on Alexis or other girls.
I knew this was the case because i remember not only being kicked out of the truth or dare game, but Layla doesn't invite you to the hot tub the next day... so i knew the Layla path was fucked(at this point i simply dropped this save, no point in keeping it).