Most people wouldn't see the internet as a threat to their relationship.

It sits quietly in their homes and delivers food, sorts out birthday presents and keeps them in contact with friends. It assumes a role not dissimilar to a trusted supermarket, open all hours. But if it's like having a department store in the living room, it's one with a sex shop as an optional extra. And for an increasing number of the couples counselled by Relationships Scotland, the internet has become an unwelcome third party in their relationship.

Pornography has been around for thousands of years; the difference now is that it is freely available. You can move from booking a holiday to online sex in a couple of clicks. A few decades ago, accessing pornography usually meant buying a magazine off the top shelf or asking for an under-the-counter video. Now, you can access it in your own home. Internet pornography poses a threat to our most intimate relationships. Intimacy is about trusting your partner with the most vulnerable parts of yourself. It's what makes each partner special to the other. Sex can be a part of it, but it isn't the only part. Knowing one another and how your relationship works creates stability; and stability is what holds and nurtures the family.

Relationships Scotland is the largest couple counselling agency north of the Border, and over the past five years, there has been a increase in couples and individuals coming to us saying their usage of porn is either out of control or is affecting their relationship. We are not talking about occasional usage: these people are spending hours every day accessing porn online. They have moved away from the intimacy of the couple and family and drifted into the intimacy of the internet. Sometimes, they have come for counselling for issues unrelated to porn. Perhaps their relationship isn't as close as it used to be, sex isn't as frequent or one partner is asking for things the other isn't happy with.

Our counselling experience suggests that online pornography is something people drift into, rather than something they plan to become involved with. They stumble across something and are curious. Maybe the couple are not getting on; perhaps there are pressures – a new baby, long working hours, money worries. Not knowing how to deal with their problems, they may find themselves drifting apart: arguing more often, retreating from each other. In the absence of anything else to do, they'll surf the net. As a method of soothing pain, going online may seem easier than facing your partner and saying you are not happy.

It starts as a distraction then becomes a reliable friend, always available with a never-ending array of experiences. It isn't just porn: chat rooms, fantasy games can all provide access to another dimension where feelings are soothed.

Like drugs or alcohol, it takes you away from difficult feelings. Only afterwards, it might not feel so good. Users often report feeling shameful and guilty, and there the cycle starts: you feel bad so you use the internet to feel better and then you feel bad so you use the internet. Hooked. By preventing couples from dealing with their problems, pornography can have lasting effects on relationships and families. Imagine the impact on children in a family where parents deal with difficulties by retreating online? Between 25% and 50% of the couples who seek help from Relationships Scotland have been using the internet as a way of managing the difficulties between them. They may not all be watching porn; they might be involved with a fantasy football league, looking up old friends, bidding in online auctions. Any excessive Internet use can interfere with a relationship.

Pornography, however, brings particular difficulties. It is usually a secret, rather than a private pleasure. People tend to keep it hidden from their partners and secrets have the potential to harm relationships. If sexual expression is gained through someone other than our partner – even if the contact is virtual – then that is relevant. When all your needs are being expressed online, your partner is being excluded. And when a partner finds out what's been going on – as they usually do – they can feel just as betrayed, humiliated and devastated as if they'd discovered an affair. Relationships Scotland has decades of experience in working with couples to help them repair the damage wrought by infidelity. However, online porn creates different problems.

Young people, in particular, may come across sexual practices online that they might not have known about. People have always been curious about sex, about what it is and how you do it. The internet gives access to a lot of views and practices; not all of them healthy. As counsellors, we are increasingly working with younger couples who are very experienced sexually, but find intimacy difficult. It's easy to have sex; less easy to form a relationship. Problems can also emerge when someone assumes that pornographic practices are normal, and wants their partner to behave like the people they've watched online.

While some element of sexual fantasy is fine in relationships, it might be that people are being coerced into acting out or behaving in ways they wouldn't really want to. Sex should never be based on coercion or doing things you are not happy with. Our therapists are very familiar with the skewed sexual response cycle that can develop when people use pornography. Rather than a shared, sensual activity, sex becomes something that is purely personal, focused on the viewer's arousal and satisfaction.

When that happens, our counsellors help couples to re-establish communication and intimacy, rebuild trust and closeness, and redevelop a more equal and mutually satisfying sexual relationship. The dilemma we face is that today, sex in all its forms is instantly available in our homes. Parents, in particular, need to be aware of the dangers posed by the Internet.

The Government's proposals to restrict access to online pornography may be a step in the right direction. Those who really want to view will continue to do so – an opt in/opt out won't stop them. On the other hand, these measures might encourage people to talk more freely about what they want from the internet and if couples can have a conversation that is open and honest, it might be the beginnings of them setting clear boundaries about its usage and potential to abuse. Perhaps there should be health warnings on all computers, just as there is with tobacco and alcohol. After all, the internet's potential to harm is just as great.

Anne Chilton is head of professional practice (counselling) at Relationships Scotland. She is also a practising relationship therapist and a qualified sex therapist