It is the age old bedroom dilemma: Do you get down to it, get it over with - or simply not bother at all?
Now, Canadian researchers say it's not as black and white as whether you indulge or not - and the ultimate reason you decide to have sex could heavily influence how happy your relationship is.
They say in most cases, it's better have sex than avoid it - even if you're not in the mood.
That's because according to new research, we're happier in our relationships on the days we have sex.
But, more than that, there are two main reasons why couples in a long-term relationship or marriage have sex, the Wall Street Journal reports.
'One thing we wanted to know is whether it really matters to your partner why you want to have sex, as long as they are getting what they want,' Dr Amy Muise, a psychologist from the University from Toronto, told the WSJ.
One is positive (an 'approach' goal) - such as to feel closer to a partner. The other is negative (an 'avoidance' goal) - basically to avoid an argument or reduce feelings of guilt.
The categories are then subdivided again into self-focused or partner-focused, Dr Amy Muise explained to WSJ journalist Elizabeth Bernstein.
Dr Muise and her team asked 108 couples to keep diaries, charting daily their levels of desire and sexual satisfaction each day. They also had to list the reasons why they had sex over a two-week period.
After they had sex, each partner answered a questionnaire about their motives - ranging from 'to make myself feel better' to prevent their partner from feeling rejected and upset. Each was rated on a sliding scale of importance.
The researchers found that when sex is wanted for a positive reason - such as to increase intimacy - then we feel happier and more satisfied in the relationship - and this transfers to our partner, increasing their satisfaction, too.
But having sex out of guilt doesn't bring as much sexual satisfaction - and even if you think you're doing your partner a favour, your level of desire will affect how much they enjoy it - and how happy they are in the relationship.
And contrary to what some people might think, men have guilt sex too.
'Men do have higher desire in general, but the motives for sex and the way they make people feel aren't different for men and women,' Dr Muise said.