“There is a flame
that flickers and sways
But burns while we’re apart”
Tear Your Love Apart
The goal in marriage should always be a sex life in which each spouse is satisfied by both the quality and quantity of sex. In fallen world there may be occasional obstacles to this (trauma, sickness, conflict, etc) but this is the goal.
Sex is an opportunity to both enjoy and to serve the other in a way that is powerful and unique to marriage. If you desire to have sex three times during any given period of time, but your spouse desires to have sex five times during that same period of time, then you have the opportunity to enjoy sex three times and lovingly serve the other two times.
To withhold physical affection from the other in marriage (be it sexual intercourse or a back rub)is selfish. According to Scripture withholding sex and not seeking to satisfy the other is sin (1 Corinthians 7.5). Our bodies are NOT our own in marriage. They belong to one another. This affects not only the frequency of sex but the way in which it is enjoyed. They may do nothing for you, but if your wife wants a back rub, get to it!
Sex is to be given and enjoyed. You both need to talk about this issue openly and graciously with one another as very often the frustrated spouse tend to suffer in silence. When there are deep issues of pain involved there may be need to get outside (pastoral counsellor, therapist, etc.)