According to the FreeDictionary.com, fidelity refers to "faithfulness to obligations, duties, or observances" or "exact correspondence with fact or with a given quality, condition, or event." Fidelity, then, certainly refers to faithfulness, but goes far beyond not cheating on your wife.
A faithful spouse would be a spouse who faithfully meets all obligations and duties. Surely that includes sexual obligations, but it goes so far beyond that. True fidelity is an exact correspondence to the qualities and conditions of marriage. Are you living up to the qualities and conditions of marriage?
Consider, for a moment, the imagery of Ephesians 5.
22 Wives, be subject to your own husbands, as to the Lord. 23 For the husband is the head of the wife, as Christ also is the head of the church, He Himself being the Savior of the body. 24 But as the church is subject to Christ, so also the wives ought to be to their husbands in everything. 25 Husbands, love your wives, just as Christ also loved the church and gave Himself up for her; 26 that He might sanctify her, having cleansed her by the washing of water with the word, 27 that He might present to Himself the church in all her glory, having no spot or wrinkle or any such thing; but that she should be holy and blameless. 28 So husbands ought also to love their own wives as their own bodies. He who loves his own wife loves himself; 29 for no one ever hated his own flesh, but nourishes and cherishes it, just as Christ also does the church, 30 because we are members of His body. 31 FOR THIS CAUSE A MAN SHALL LEAVE HIS FATHER AND MOTHER, AND SHALL CLEAVE TO HIS WIFE; AND THE TWO SHALL BECOME ONE FLESH. 32 This mystery is great; but I am speaking with reference to Christ and the church. 33 Nevertheless let each individual among you also love his own wife even as himself; and let the wife see to it that she respect her husband (Eph 5:22-33)."Yeah, yeah, we've seen that before ... too many times." Now, wait. Don't jump too quickly. Assuming that you just skimmed that rather than read it, I'd like to point out some pertinent points.
There is a popular argument today that says, "Wives don't need to submit to their husbands. Ephesians 5:21 speaks of mutual submission." Fine. You go with that. But there is a key concept going on here that you may have missed. The key concept is found in verse 32: "I am speaking with reference to Christ and the church." There is a mystery involved here where the relationship of Christ to the Church is paralleled by the relationship of husband to wife (and vice versa). Thus, the qualities and conditions of marriage are demonstrated in the relationship of Christ and the Church.
We read, in that light, that wives (as symbols of "the Church") are to "be subject to your own husbands, as to the Lord" not in a vacuum, but in the same way the Church is subject to Christ (v 24). We read, again in that light, that husbands (as symbols of "Christ") are to "love your wives, just as Christ also loved the church and gave Himself up for her." Thus, just like Christ and the Church, wives submit to husbands and husbands give themselves up and love their wives. (Do I really need to explain further, husbands, how Christ very much gave Himself up (Phil 2:5-8)?) Mutual submission? Sure. But not the same type of submission nor the same outworking. But key here is that the biblical qualities and conditions of marriage include 1) the imagery of Christ and the Church along with 2) a wife who submits to her husband in everything as to the Lord and 3) a husband who surrenders himself and loves his wife.
Now, look, these are just a few of the biblical concepts surrounding marriage. My point here is not to expound on all the qualities and conditions of marriage. My point here is fidelity. Marital faithfulness goes far beyond sex. It even goes far beyond lust (Matt 5:28). It goes well into how wives are to respond to their husbands and husbands to their wives. If you are not doing what the Bible commands must be done, you are being an unfaithful spouse. That is, if you are not fulfilling the obligations and duties of marriage and are not accurately reflecting the biblical qualities and conditions of marriage, you, my friend, are an unfaithful spouse. Probably something you might want to look into.