Most people are happy to discuss - often in the most intimate detail - every aspect of their love lives while they are dating.

But as soon as a relationship becomes serious and two people become 'a couple', the shades are swiftly drawn against.

The world and that loyalty and confidence gets switched to their partner. From then on, we rarely see a glimpse or hear about what's really going on in their life as a couple.

It's only natural to want to confide all intimate secrets and thoughts to your partner rather than your friends.

But unfortunately there is also an unspoken pressure on couples today to 'live the dream' and present a happy united front of commitment, romance and fidelity.

Admitting to disappointments or unhappiness within your relationship can be seen as failure, so people don't talk about them - usually until it's too late. 

But the fact is, we really do need to know how people live in lifelong love, how they make their relationships work, and what is going on behind those walls of secrecy, so that we can make sense of our own relationships.

Without a window into the domestic lives of others, we have no way of knowing how relationships change through the course of life, and whether we are experiencing the normal dips associated with the ups and downs of marriage - or a deeper malaise.

A committed, loving partnership is central to our health and happiness, as well as that of our children. By throwing light on the domestic lives of others, and looking at some of the best relationship research, I hope to correct some of the myths about the state of modern relationships and reassure you that you are probably doing far better than you give yourself credit for.

What really goes on in the bedroom? Good sex is the glue that is supposed to hold marriages and relationships together. Bad sex, boring sex, or no sex at all is considered a good enough reason to leave.
For centuries sex was compromised by a range of issues, from fear of pregnancy to venereal disease and the power of the Church.

Now most people feel we don't have to marry to have sex, we can experiment with a range of partners before we settle down. However, new sexual pressures and expectations have surfaced as a result - which throw up new problems and compromise lifelong relationships and our attitudes to fidelity, commitment and happiness.

Sex was inevitably the one great area of difficulty in the countless interviews I conducted for my research. I was aware of having to brace myself to ask questions about it and push through our mutual embarrassment to try to get at some sort of truth.

Surprising numbers of couples in long-term, stable relationships said that they could talk about anything to each other, but then went on to say for the first time in each other's company that they were aware that one of them - always the man - wanted more sex than they got.

But without honesty, we will inevitably compare our relationships with a romanticised, mythical image of explosive, sexual compatibility and end up feeling inadequate.  The truth is, sex is occasionally messy, painful, unwanted or boring.  For most people, sexual desire fluctuates through the course of a relationship, and reflects how we feel about ourselves and each other.