"One myth I particularly want to challenge is that after the first few years it's downhill all the way and once you get past 40 that's about it - you've got one last chance and you'd better grab it quickly. That encourages all sorts of stupid affairs.
"However, if couples make love rarely it leaves the relationship pretty vulnerable, because we don't lose our need for sex. It's a wonderful way of feeding a relationship. It's not just about orgasms: what's particularly restorative is that afterglow, where you hold each other and feel cared for. But if you don't feed your relationship it dies, or someone else comes along and feeds your partner. I don't think people get divorced because they have a bad sex life, but I certainly think it's a contributing factor."
Marshall encourages couples to reinvent their sex lives every few years. It's not about spicing things up superficially with new techniques and toys but about building confidence and openness. If couples can pull this off - in the face of undeniable pressures like kids and careers - sex gets better and better. Yet the very glue that binds long-term relationships can hamper progress, because individuals are naturally wary of suggesting changes for fear of rocking the emotional boat and as time goes on there's so much more at stake. And while it's all very well for sexperts to bang on about the importance of communication, most couples haven't got a clue where to begin.
Too often sex has become the elephant in the room; a subject far too scary to bring up because it feels like criticism. So much easier to bite your tongue and put up with things the way they are.
Marshall's advice is to avoid bringing up problems which will make your partner feel defensive. Instead start by talking about what you like about your sex life and remembering what was wonderful in the past. That should to break the ice for further discussions about how to bring more good stuff into the relationship now.
Marshall is also keen to bust the myths about sex which hold couples back: that it has to be spontaneous and that both partners have to be equally turned on at the same time.
"That puts people under extreme pressure," he says.
"What's needed is a bit of give and take and accepting that sometimes one person is in the spotlight, sometimes the other. If you wait until you both feel in the mood you'd probably only have sex once a year, on holiday. That's not to say you can't have spontaneous sex, just that you can't rely on it. The rest of the time you need to plan."
And he urges couples to treat sex as a priority, rather than the last thing on the minds of two exhausted individuals. Parents, whether their children are teenagers or toddlers, should take note: "If anything is causing problems in our sex lives, it's the sense that we have to be super-parents who are available to our children 24/7," he says.
"I can't tell you how difficult it is to persuade couples to put a lock on their bedroom door, although they wouldn't dream of barging into their kids' bedrooms! If your kids hear you making love, Hurrah! It says you are sexual creatures and I think that's incredibly reassuring because it gives children the message that their parents love each other - and that is a wonderful bedrock for them to have."
Sexual Healing:
* Take the pressure off by having a break from sex for a few weeks. Focus on touching instead.
* Develop habits that give you a head start, like going to bed at the same time as your partner and keeping distractions such as computers and phones away from the bedroom.
* Simple communication also helps: if you're going to bed, then make a point of telling your partner, so they know you haven't just gone for a bath.
* If you've got children, put a lock on your bedroom door. If you're worried about being overheard, play music.
* Don't wait to be in the mood. Sex doesn't always have to be spontaneous. Plan sex.
* Communicate. Bringing up the subject of sex can easily be taken as a criticism. Don't focus on problems but talk about what's good about your sex life and what you enjoyed in the past.
- INDEPENDENT
