Depends.
You used the phrase 'found out', which implies that she didn't tell you (or in this hypothetical case, me).
I'd be pissed of because someone I trust obviously didn't trust me enough to tell me the truth & I was thus unable to avoid porn that featured her (although it's highly unlikely that I'd watch porn featuring my own mother, since I'm not interested in old porn (more than 10 years old) & I'm not into cougars either). I wouldn't stop watching porn though, but I would avoid the photo shoots/scenes/movies featuring her (mostly because I don't consider porn or other sexual activities featuring/that include family members to be very exciting). The same would apply if you asked the same question but changed mother into daughter.
Anyway, I would choose potion 4, but I don't know what my imagination has to do with it.