Does your wife feel you want her sexually, or does she feel you want sex and she’s your only choice?
When you say: “I’m horny, let’s make love.“
Does she hear: “I have an erection and you’re the only place I am allowed to put it.“
Many women in our culture have the idea men think of sex much as they think of going to the bathroom – it is a basic need and you just do it whenever and with whomever you can. (Sadly, there are some men who seem agree.) If that were the truth, then it would be logical to think a good Christian man is the same way but limited by rules saying he can only have sex with his wife. Does your wife think this way? Maybe she knows better but still feels this way on some level.
Let’s face it, feeling you are the only non-sinful receptacle for your husband’s penis would not exactly be a major turn on. It would not make a woman feel loved, special, or desired. Even if you’ve never said or done anything to suggest this is your reality, your wife lives in a world that assumes this is the truth for many (or most, or all) men. Odds are she has some fears and doubts along these lines.
What you can do: A frank discussion would be great, if you can do it. With or without such a talk, be more aware of what you say and how you say it. Work to communication you want HER, not just a few of her body parts. Help her to understand you don’t have to control yourself not to have sex with anyone else because you do not want to have sex with anyone else.