Pre and post marital sex are such different things they should come in completely separate, not sub, categories. And then there's post-children sex... an entirely different, erm, ball game....
Show me a couple who have been together for more than five years and are still having inventive sex on a regular basis and I'd bet my favorite handbag they're getting some form of external assistance; yes, I'm talking the full smorgasborg of carnal delights, the kind of stuff Channel 5 obsessively churns out tediously titillating documentaries on.
Show me a couple who have been together for more than five years, have toddlers in the house, and are still having inventive sex on a regular basis and I'll eat my favorite handbag (Italian leather and very chewy, that's how sure I am about this).
Let's face it, you've been with the same person for a thousand years, you know every single button to push, and you know the formula for what gets you both off in the quickest time in order to avoid the possibility of being interrupted mid-shag by a wide-eyed and possibly disturbed-for-ever child - who then spends the next three years telling the neighbours about the night Daddy took his clothes off and beat Mummy up in the middle of the night (don't take that as an admission of strage sexual practices in our household, just think about it from the perspective of a three-year-old).
So, it stands to reason that the days of re-enacting that scene from 'The Postman Always Rings Twice' is going to be a distant memory, something to be pulled out of the dark recesses of your mind at the most inconvenient and ultimately unfullfilling moments, such as when on the tube or in the middle of an annual performance review. Then add the fact that your other half has experienced you in the throes of childbirth and you're getting into intensive therapy territory. Oh yeah, and not having time for the pre-child(ren) intensive wax, blow dry and general beautifying routine can't help much; hairy calves, hoof-like feet and enlarged pores are not conducive to acts of mind-blowing lust.
So, what can you do about it?
Well, apart from hiring round-the-clock nannies and an unshockable beautician ("You had your last bikini wax when?"), I'm not sure there is an easy solution - especially if your temperament isn't suited to group sexual activity (and from what I've seen on TV, participants in this particular pecadillo tend to have similar levels of physical attractiveness to that of the House of Lords).
But there is hope; apparently you can expect a vast improvement in the quality and quantity of your sexual activity when the kids leave home. I plan to start buying shares in Viagra now.