As a Christian, we are supposed to give ourselves to our spouse, in fulfilling our marital obligation.
As a practical matter, people who routinely avoid sex with their spouse, often end up with unhappy marriages. The bitterness and lack of affection, will spill over into other things. People getting angry in their marriages over really dumb problems, in many cases in actually the result of the lack of affection in the bedroom. It's hard to be bitter over stupid issues, with someone you are routinely extremely intimate with. Not so much, when you are rarely intimate with them.
The other side, is that at some point, if the husband doesn't ditch the wife, he'll eventually switch off. He'll just shut down. This is why 20% of all married couples, live in sexless marriages.
Now of course there are other factors, like physical problems, that prevent sex. But generally, when a girl shuts down her husband enough times, he'll switch off. Then later in life, after the kids are largely out of the house, or gone, suddenly she wakes up sexually again, but he couldn't care less. He's been switched off for so long, it's not even on his radar.
This is why you see movies like Hope Springs, where the disconnected husband, and the sex starved wife, desperately seeking the attention she got when they were younger.... and duh... having sex.
Where do you think the Cougar phenomenon came from? These older chicks switch back on, while their shunned husband does not.
My opinion? And this is just my opinion, so who cares what I think? I think either spouse should never turn down the other to have sex. Unless obviously, one is being abusive or, you actually are sick, or one is hooked on porn, and wants some sick twisted crap. But otherwise, you should take up your spouse every time they want sex.