Marriage
Consider this spiritually inspiring quote from Ephesians 5:33: "each one of you also must love his wife as he loves himself, and the wife must respect her husband."
Sex is a healthy and integral part of the relationship between a Christian husband and wife. It is not, however, something that either spouse can demand from the other whenever they feel like it. Christian married couples must always cultivate a healthy respect for proper timing and never pressure their spouse into sex.
Some people use an unfortunate interpretation of 1 Corinthians 7:3 to argue that Paul the Apostle said sex must always be offered up if it is asked for. The verse in question reads "The husband must fulfill his duty to his wife, and likewise also the wife to her husband. The wife does not have authority over her own body, but the husband does; and likewise also the husband does not have authority over his own body, but the wife does."
The problem is that interpreting this verse to say that it is acceptable to pressure one's spouse into sex betrays a misunderstanding of what true, Godly authority looks like. Authority is not about doing what you will with no regard to the feelings of those you have authority over. Have Godly authority over the body of your spouse means treating them with compassion, respect, understanding, and never forcing them to do what they do not want to do.
You do have a right to have your sexual needs fulfilled, but not to demand that they are fulfilled exactly how and when you want. If you require any further clarification on scripture from this issue, then consider this spiritually inspiring quote from Ephesians 5:33: "each one of you also must love his wife as he loves himself, and the wife must respect her husband."
Having love and respect for your spouse means respecting when they are not in the mood to have sex and showing them love regardless. It doesn't matter if they are too tired, not feeling well, or simply not in the right mindset, you should not make them feel badly if they reject your advances. Let them know that they are your number one priority and that you simply look forward to next time.
It certainly is an issue if your spouse never wants to be intimate. As Paul the Apostle says in the quote from 1 Corinthians above, husbands and wives do have a duty to each other and should not withhold their bodies from one another. There is a difference between feeling tired or sick once in a while and never having the willingness to be intimate with your spouse. Even in this circumstance, however, you should never pressure your spouse into sex. Talk to them in an understanding and compassionate matter and discuss what the two of you can do together to rekindle your sex life.
Sex is an amazing, life-affirming, pleasure-giving, and relationship-strengthening act. Do not take these wonderful aspects of sex away by refusing to respect proper timing. Sex within a healthy Christian marriage should be something that both spouses enthusiastically take part in and find enjoyment in.
Insight from Scripture
For all that is in the world—the lust of the flesh, the lust of the eyes, and the pride of life—is not of the Father, but is of the world. - 1 John 2: 16