Value

We value things that are rare and conversly things that are abundant are generally of low value.  Diamonds, Gold and so on are valuable because they are rare.  Air, soil, old cars - well they are in abundance and so are considered as low value. 

Another thing that drives value is the difference between supply and demand.  When food is scarce then it is expensive because the demand or the need is high but the supply is low.

Men tend to want sex.  A lot.  That is called "a need".  If the supply is low then the price or value placed on it goes up.  The price translates to effort.  As a sweeping generalisation, before marriage, sex is rare and so the man (or boy) will put in a lot of effort.

However once married, the wife may think, as mine did, that it is part of her obligation to provide her husband with sex.  And for a while that sustains him.  As it did for me.  In fact initially I could not believe my good fortune.  We had both come from traditional Christian families and we had met through the Church and we continue to be active members.  So for me it was sex sex sex.  And without realising it, the value for me went down down down.  No longer did I have to expend effort to get sex.  I just had to say to my wife that I wanted sex and she would, without complaint, give it to me.  At the same time, she was also attaching a low value to sex but in a slightly different way.  It was seen as a sacrifice.  Sometimes painful.  Certainly not rewarding.  Not something to be valued and therefore not something to which she would exert effort to obtain.

So if sex is rare then the value will be high.
For him, he has to exert effort and that comes through waking up and reading his wifes needs.  And of course not doing the things that put her off.