Let me start with this, how do you ask her for sex? This is very important. Don't just admire her and then tell her "Dear, I want SEX". I believe you are not doing something right, that's why she is restricting you. Other reasons might be :
1. you are not good in bed, probably short manhood or quick ejaculation.
2. maybe you are not available too when she wants it.
3. maybe you just swing to action without caressing or touching some sensitive part of her body, like tickling her or fingering her.
4. you might be rushing the sex, so she might not be interested.
5. maybe you are not Romantic.
This is very important.
I remember 3years ago, I was with a girl in the midnight, I just woke her up, and said, " I want sex" and she replied "am not in the mood". And she turned her back towards me. I was embarrassed, after some minutes she turned to be and said " OK let's do it, but finger me first" Then I realised my mistake. Having sex with a girl when she is not in the mood is kind of tough, you need to be experienced.
You don't ask her for Sex, you should make her Want Sex.
And again, some men only turn to their wife when they want sex, if am that lady... I swear I will make you want it so badly, and then deprive you from it, just to punish you, but I will later give you sha.
Make sure anytime you want to have sex with your wife, make sure you are romantic, cuddle her even if its difficult, say crazy stuffs to her, and don't forget to caress her, she will pave way for you even before you say " babe, I want Sex" you would have been done with the knacking, before she realized the strategy you used to make her open up.
If she say, work is taking her time and stress is too much, then start demanding for sex during weekends / when she is off duties, your chances of having it is 70%.
So bro, check which area you are falling out and make amendments.

The best is to have excess money and she will always be interested in giving you sex so you won't look outside.

dheaven:
The best is to have excess money and she will always be interested in giving you sex so you won't look outside.

I Hail you bros