The most obvious passage on the topic is in Paul's epistle to the Ephesians.
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 (Eph 5:25-30).There is one key phrase: "Husbands, love your wives." Absolutely essential. Fail at that and you fail at marital fidelity. Now keep in mind it is agape love which doesn't love with return in view. Indeed, isn't that what Paul said? "As Christ loved the church." That is, "while we were yet sinners (read "hostile to God") Christ died for us." It is a "gave Himself up for her" love. It is a love that includes cleansing her "by the washing of water with the word". Is your wife, because of your ministry to her, "holy and blameless"? It's part of your marriage conditions. It is an "as you love your own body" love. You nourish and cherish it. Do you nourish and cherish her? This isn't "the optimum marriage". It is part of the basic qualities and conditions of marriage to which all faithful husbands are required to conform.
The other definitive passage on the subject is found in Peter's first epistle.
You husbands likewise, live with your wives in an understanding way, as with a weaker vessel, since she is a woman; and grant her honor as a fellow heir of the grace of life, so that your prayers may not be hindered (1 Peter 3:7).Don't let the fact that it's one verse cause you to miss what's there. It is a truism that men know nothing about women. It is not supposed to be true. Indeed, husbands ought to be students of their wives. No one should understand your wife, husband, better than you do because that is part and parcel of what is expected of you in marriage. Husbands are required by God to be the head of the house (1 Cor 11:3), but not as lord and master. No, she is to be treated with honor, like fine china. Notice that it does not say that she is a weaker vessel, but that you must treat her as such. Note, also, that this particular condition of marriage carries with it a severe warning: "That your prayers may not be hindered." Having trouble praying? Is it because you are not a faithful husband in this regard?
There is an annoying passage that is pertinent to the topic. I am addressing this passage here because it is husbands I have in view. Paul told the church at Corinth, "Let the women keep silent in the churches; for they are not permitted to speak, but let them subject themselves, just as the Law also says. And if they desire to learn anything, let them ask their own husbands at home; for it is improper for a woman to speak in church (1 Cor 14:34-35). I know, I know, the women don't like that one and they'll likely get upset, but what I'm aiming at here is the responsibility of husbands: "if they desire to learn anything, let them ask their own husbands at home." Husbands, are you prepared to teach your wives whatever they may wish or need to learn? If you are going to be faithful to your marriage, it is required.
I can't leave this topic without touching on the sexual ethic. We already know that sex outside of marriage is sin. Any form of that is infidelity. Got it. But the Bible is not silent beyond that.
Let the husband 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. Stop depriving one another (1 Cor 7:3-5).Another truism that is that many (most?) husbands don't get the sexual satisfaction they desire in marriage. Please note, husbands, it's not your job nor your right. Your job is to fulfill your duty to your wife. Your job is to provide satisfaction and joy and pleasure to your wife. You do not have authority over your own body. You are not the primary issue in the marriage bed. As long as you miss that point, husbands, you are failing to be faithful husbands.
Marital fidelity is much, much bigger than not cheating on your spouse. Jesus took the first obvious step in that direction by assuring us that lusting after another woman was adultery. You may classify yourself as sexually faithful, but if you lust after another, you'd be fooling yourself. But beyond that, fidelity includes all aspects of marriage. It includes the basic unity that defines marriage (Gen 2:24; Matt 19:4-6). It includes leaving and cleaving. It includes husbands loving their wives in a sacrificial, selfless way. It includes honoring your wife and understanding your wife. It includes being the leader in the home which itself involves being the spiritual leader and the primary teacher. And there is the whole aspect of sexual fidelity that views her sexual needs as more important than your own. All of this is part of marital fidelity, men. How are you measuring up?