Decreased sexual appetite is quite normal. It happens to everyone, both men and women, but mostly to women - I don't know why... I never did and I never will. Women are very strange from this point of view. It happens to men, too... it happened to me. Sometimes I just don't feel like having sex for a few days in a row. I just don't...
Your life style
Low sexual appetite may be triggered by a sedentary life. The moment you get caught in routine, you get less active and bored. Sedentary life decreases stamina, thus the usual appetite for a quickie goes away. This usually happens to middle aged men and that midlife crisis happens just because of this. Men want to escape the routine and sometimes they do stupid things (cheating) or go for a grown-up alternative: engage into something that would spice up their life or the relationship.
Depression
Depression is another cause for low libido and this one can affect all men, regardless of age. A depressed man is considering life itself - if it is worth living or just quit, sex is no longer a priority for him. Depression also reduces energy and self esteem, two essential factors that produce sexual craving physically and mentally. Moreover, even if that man would consider sex, depression is not a turn on - or at least not for most of the women out there.
The solution is to get rid of depression somehow and if you're considering antidepressants, think again. Antidepressants reduce sexual performance and they will produce what is called a late orgasm if none at all.
Exercise
If you really want to do something about your sexual appetite, exercise on a regular basis. Not only that you will improve your overall health, but getting fresh air, interacting with people, seeing people outside, the warmth of the sun, the life and activity outside your apartment will definitely make you feel a lot more active, cheer you up a bit, make you forget about stress maybe and give you more energy for sex.
Eat healthier, and if possible more food rich in zinc (oysters, beans, nuts, meat, etc) to increase the production of sperm. You don't need to be a genius to figure it out: more zinc, more sperm, more sexual appetite. Along with an increased sperm production, your body will produce more testosterone as well, which also increases sexual appetite.
Penis enlargement
A new exciting project that will take your mind off your daily worries, break the routine and give you a bonus: a bigger dick. A penis enlargement program will also increase your confidence along with your penis size because sex passes first through the brain. I don't think you can achieve a good, hard erection without mental stimulation.
This new exciting project can bring you two closer if you know how to express yourself. Let her know that you need to do something about your condition and that you need her there for you to support and understand, maybe even measure the results for you. This new thing will be your secret, a new confidence test, a reassurence that you are a strong couple. A new secret project will even increase the thrill in bed as you will want to test the results and see how she reacts to your new, tuned up penis.