Long gone are the days that marriage would guarantee frequent free sex. In fact it's the opposite now. To guaranteed sex you should stay single or keep moving between short term relationships. Once the relationship becomes long term then the offer of sex quite often stalls and is replaced with the risk of taking the house and kids instead.
Those days never happened, simply in the past, more men were Alpha and fought against the betaisation process aka the first successful denial of sex. The wife knows that if she wants to keep her husband happy, she'd better make the marriage pretty sweet, and that means providing lots of sex. However, the further down the curve the husband goes, the more invested in her he has become, and she realises 1 day that she can deny him sex (often due to a seemingly good reason aka headache, period pains etc) and the husband won't leave.
So far he accepted marriage based on the idea that he will get sex on tap. Now the well is drying up. If she successfully denies him once and he hasn't left her, the behaviour of withdrawing sex whenever she wants has begun to be conditioned as an acceptable behaviour. It will start out for "valid" reasons as yours truly explained above, but when she sees that it affects him emotionally, the idea that she can use withdrawal of sex as a weapon against him emotionally will be instilled and denials will increase in pace over the next few months. Slowly at first, the beta don't really notice or mind much. But it *WILL* pick up pace. She now has control over 1 of the main things that really got him in the marriage in the first place. However now he is so emotionally invested, he has other things at stake and will want to stay with her even with sex drying up.