Because you're not designed to physiologically.
Only one quarter of all women orgasm through penetration despite both men and women believing the opposite: that most women do.
Dr Laurie Mintz, a US psychology professor and human sexuality expert (and author of Becoming Cliterate), thinks we've separated our most reliable route to female orgasm - clitoral stimulation - from how we feel we should orgasm - penetration.
The result is men have at least three times as many orgasms with a partner than women do.
Why am I bored by sex, even though I love my partner?
Ironically, the couples that are the closest are the most likely to suffer loss of desire.
The qualities that love loves - reliability, affection, routine - lust hates.
Lust thrives on danger, eroticism, the unknown - things that make us feel uncomfortable.
It's hard to maintain both and because we spend more time hanging out together than we do having sex, most couples opt for being in love over being in lust.
There are some couples who are having hot, rampant sex after a decade together, but it's rare, not common.
The trick (and you know what's coming - even if you aren't) is variety.
Do the same thing all the time, like most couples do, and of course you will be bored.
Remember the kind of sex you had at the start that you both loved?
Write down, in as much detail as possible, what you did.
Sex in a semi-public place? Quickies? A day spent in bed with lots of pre-planning on what you'd wear and do?
We put more effort into our sex lives than we think at the beginning: it's not all hormones.
Each come up with 10 things you'd like to explore again and try one a fortnight.
If you're lost for inspiration search "new things to try in bed" - you could spend days reading all the suggestions.
Is there really a G-spot?
It's certainly not the "magic button" everyone thought it was going to be.
But on some women, there is an area around one to two inches along the front vaginal wall (the bit directly under your tummy) that can produce intensely erotic feelings.
The key word here is '"some".
Some women find G-spot stimulation highly uncomfortable or "just weird".
The G-spot remains the Marmite of sex: some are bonkers about G-spot stimulation, others can't bear it.
Am I gay if I have fantasies about being with a woman?
No. Most people fantasise about things they have zero desire to play out in real life.
Women are more likely to have same sex fantasies than men are, probably because it's seen as more acceptable.
We're also less likely to get hung up on homophobic fears that we're gay.
The woman featured in our fantasies usually isn't someone we know, but she always seems to know exactly what to do to turn us on because she's got what we've got.
Some psychotherapists believe same sex fantasies are narcissistic: we're imagining what it would be like to make love to ourselves.
I think they're more about healthy sexual curiosity: it's something we all wonder about and something men encourage, given their obsession with two girl, one guy threesomes.
Why does he prefer having solo sex watching porn than having sex with me?
Because porn sex is effortless sex.
He can turn on his computer and be treated to millions of different images, focusing on hundreds of different themes, to suit every taste and every mood.
There's no judgement, no embarrassment if his erections aren't as strong as they used to be, he's never bored and it can be all over in four minutes - start to finish.
Having sex with you requires being nice in and out of bed, being sexually skilled and you wanting sex the same time as he does and the same type of sex.
Tell him, calmly, that you know he watches porn and say you want to know what appeals to him most about it.
Take your cue from there.
We have mismatched libidos. Are we doomed?
No you aren't but you are in for a rocky ride.
Couples often don't find out just how different their libidos are for about a year because new sex artificially inflates the lower sex drive person's desire for sex.
It's when all the sex hormones wear off and cuddle hormones like oxytocin move in that you find out one of you is up for it morning and night and the other would quite happily settle for once a month.
Fixing differing sex drives generally involves coming up with some sort of compromise over how often and how long you have sex.
Something that I've found works well with couples is both agreeing on a number of five or ten minute sex sessions per week.
It satisfies both: the high sex person gets sex more often, the low sex person doesn't have to commit to long sessions.
Putting a time limit on it increases intensity.
How do I get my libido back after having a baby?
It's not just women who experience loss of desire after a baby: lots of Dads do too.
Babies mean sleep deprivation and being emotionally and physically there for another little human being pretty much every moment they're awake.
Not exactly the right head space for sex.
Loss of libido is very normal with lots of couples saying you don't really get your sex life back for two years - and that's if you're lucky.
Help it return without putting pressure on yourselves by staying tactile and doing sensual rather than sexual things like having a bath together and giving each other massages (without anything expected in return).
Later, progress to low effort sex - one of you gives the other oral or hand pleasure, then reverse the next time.
Keep sessions short and low stress: even a good, long, sexy kiss can be enough to keep you erotically connected.
Most important, talk to each other about how you're feeling and reassure each other that while sex is not important right now, it will be later on.
For more non-judgemental, practical advice about sex, visit traceycox.com and lovehoney.co.uk for her range of products.
