Every single day of the week, I log into my GS mailbox to find questions on sexual frequency. "We don't have sex," "He always wants sex," "He never wants sex," "He can't get it up, is it my fault," "He is useless at sex."
We all get them, all the time. Seriously, if we had sex every time a question like this came in, we'd be pretty sore, to say the least.

I have discussed the three year rule before, but just to remind you of it, the rough rule of thumb is:

The frequency you have sex halves for every three years you are together.

Totally normal, though somewhat saddening for people who have been together for over 15 years. Oh, it doesn't have to, you can resolve to not go quiet into the night, bring the spice back into your relationship or marriage and vow to have sex once a night regardless.
I know exactly one couple who have managed that successfully. The rest who tried are all divorced.
Not going to say it was cause and effect, but when sex becomes a scheduled chore, as it does when a couple is desperately trying for a baby (for example - Carrie's question about Sex and the City put it into my head), it puts a huge amount of stress on the relationship.

Especially on the guy. I know, I can hear you all grumbling about "special pleading" and "of course you would say that," but hear me out.
There are a few things that totally kill a guy's ability to perform, along with the ratings of how likely it is your fault.

Mechanical Problems

The blood to inflate and harden your John Thomas has to come from somewhere. Any interference with a guys circulation can lead to the dreaded semi - an erection that looks respectable, but is too soft to actually use - or, worse still from both partner's perspective, the wilt, where he can only keep it up for a couple of minutes at a time.
Mostly, this is caused by high blood pressure, bad diet, smoking, a couple of drinks, being unfit, a heart problem or simple age. So, if he starts showing these symptoms, haul him kicking and screaming to a doctor for a check-up.
Or fight with him. Make-up sex, like survivor sex, is always amazing because his (and your) circulatory system is still running full throttle from the adrenaline.


Stress and Worry

There is a famous saying that men have one track minds and can't chew gum and think at the same time. While it is a stereotype, and a joke, it has some basis in fact.
Most guys are very linear and focussed in their approach to things. Which means if he has something on his mind already, sex just has to wait it's turn patiently, or, more likely, very impatiently.
The answer to this is talk. When he is ready, not before. Many guys are totally terrible at talking about their worries, due to the insistance knocked into them since early childhood that a man keeps his own counsel and sorts out his own problems. That is an incredibly hard habit to break, as are all habits learned in early childhood. Patience, understanding and a willingness to not only let him talk but to respond in guy fashion, by cutting back on the sympathy and offering solutions, will help here.


Tiredness

We all know adult life is hard. Work, the commute, fighting like weasels in a sack for promotion, studying and learning to improve your chances, the day to day chores around the house that still need to be done regardless, kids (possibly) to look after, feed and teach, aged parents to prevent from wandering around causing chaos, friends who have a nasty break-up just when you have come off a 16 hour shift - read all that and it's a wonder that any sex happens ever.


Performance Anxiety

Guys, rightly or wrongly (hint, it is wrongly) are judged on their abilities in bed. The judge is themselves, and he is a hanging judge. Tough as nails, with no excuses accepted. Fail in the sack once, and the next time there is nerves and worry that it'll happen again. In sports it is called clutching or freezing. In bed it is usually called premature ejaculation and leaving a lady hanging.
But a one off failure is not the problem here. It is the disappointed yet understanding tone you gals use that really causes the anxiety. This is one time when communication is not the key, ok? Don't say a damned word about it.


Children

Back in medieval times, if you wanted sex with your husband/wife/a prostitute, you had precisely zero privacy. If you were a peasant, you'd be having sex while his parents were asleep two feet away from you in the same room. If you were an aristocratic young woman, your first official sex act after marriage was done in front of witnesses. We don't do that now, except for thrills.
The last remaining hold over of that lack of privacy is children. The number of times you settle into bed, start cuddling, getting in the mood, starting, then being forcibly stopped by a baby crying or a two year old leaping on your bed mid action demanding you check for monsters under the bed is totally ridiculous - it seems like every goddamn time! No wonder he goes fast if you have kids. Stopping midway is frustrating as hell for you, painful and frustrating for him.
Young children are the best and most efficient contraceptive ever invented.



So there you have it. A guy's take on sexual disfunction. Read with a large helping of salt.