The dilemma I’ve lived with my stepfather for eight years. When I was a child he was kind but the older I get the more we resent each other. He can be aggressive, but what bothers me most is that he watches porn in the house, usually when my mum is out. When I was 13/14, I frequently heard him watching it on the PC very loudly. I even asked him to turn it down once (you’d think he’d get the point). My sister and I got used to walking past the living room and hearing loud female moans, confiding in each other and expressing our disgust. Now that I am 19 and she has left home I feel more alone and disgusted by it. We told our mother but all she says is, “It’s an addiction just like alcohol.”

Mariella replies What a creep. If you’d written to me six years ago, I’d have been urging you to confide in a responsible adult and even contact social services. Thankfully your position is less vulnerable now but I’m still concerned about his behaviour, which at absolute best displays an utter disregard for his responsibilities as a parent. It’s most definitely not appropriate and it’s impossible to know whether there’s also a more disturbing element at play.

Conversely, he may just have allowed himself to slide into a careless but corrosive addiction – all too accessible in the digital world. Of the great commercial beneficiaries of the World Wide Web no business has been better served than pornography. What was once a top-shelf guilty secret has multiplied into the mainstream with access for all, including the most vulnerable, at the touch of a button. This migration of extreme sexual imagery into our daily lives is something we’ve mostly ignored but in too many cases embraced – confusing cultural degeneration with sexual emancipation – and there is nothing liberating about the objectification of women’s bodies when it’s undertaken by a male-run business in pursuit of fiscal gain.

Rutting blokes pretending to give women dazzling orgasms is not a welcome formative influence in any child’s life

Just because something is accessible doesn’t make it acceptable so how a man, bringing up two teenage girls, can excuse a public, pervasive diet of porn is baffling. Unfortunately that’s also the word I’d use to describe your mother’s stance. It’s certainly an abdication of responsibility to allow him to pursue his vice so openly with two young girls in the house. Rutting blokes pretending to give grateful women dazzling orgasms in totally unlikely scenarios is not a welcome formative influence in any child’s life.

You aren’t the first child to grow up in a house with parents watching pornography but the normal routine would be to hide it from view, so it’s hard to judge the danger or seriousness of your situation. The conventional wisdom for having your PC in an easily accessible public part of the house is so that adults can keep an eye on their children’s web activities. Seen in its most benign form here we have the reverse. Your stepfather’s public indulgence is selfish, inappropriate and, when you were younger, potentially dangerous and damaging.

Thankfully, you and your sister seem to have dealt with it healthily, expressing and sharing your revulsion, disparaging his behaviour and getting on with your lives. Now you are 19 and, I hope, soon to leave for fresh pastures, so it’s hard to come up with a workable solution for an issue that’s entirely unpalatable but not illegal. How far you want to push your mother into confrontation only you can judge, or how far you yourself want to get involved in the discussion. Reopening the conversation is certainly worth attempting but a discussion with your mother about why she allows it to continue might be more illuminating. You describe him as aggressive and I’m presuming you don’t meant violent, in which case obviously you need to be consulting social services or the police.

Otherwise, their relationship, no matter how dysfunctional, is for them to resolve. You’re at a point in life now where you should be focused on shaping your future, not trying to resolve your mother’s. A last-ditch attempt at resolution with emphasis on how embarrassing it is for all concerned is worth a try. However, if, as your mum says, it proves to be an addiction that one of them is pursuing and the other condoning your chances of a positive outcome are slim. Your energies would be better employed following in your sister’s footsteps and working on an exit strategy.

Living under your parents’ roof is tough as an adult and even harder when at least one of them displays the self-restraint and inhibitions of a toddler. At the very least he should be ashamed of himself, while your mother should be assuming responsibility for her daughters’ welfare, not excusing his perversions.

If you have a dilemma, send a brief email to mariella.frostrup@observer.co.uk. Follow her on Twitter @mariellaf1