Here I just have a simple point to make: your marriage matters.
"Matters" to whom? Well:
It matters to God. Really, if I'm writing to actual Christians, we should be able to stop with this one, shouldn't we? You took vows invested with meaning He'd given to them, and He cares about that (Num. 30:2; Deut. 23:21-23; Pss. 61:8; 76:11; Prov. 20:25; Eccl. 5:5). He is invested in your marriage. He invented it, it's His institution, and you entered into it.
It matters to your spouse. You promised that man, that woman, that (s)he could trust you absolutely. Brother, you promised to lead and love and sacrifice, as Christ did for the church. Sister, you promised to respect and to subordinate yourself, as the church should do towards Christ. You told this person, "Though all else fail you, you can count on me. You can forget about looking for sex or love or devotion anywhere else. I'm your man/woman."
It matters to you. In more ways than you can think, it matters to you. Let's just start with the boneheadedly obvious: supposing you're in a marriage that is other-than-humming, and there's something you can do about it. But you're not. Why? You were going to get going on that when, exactly? When life really starts? Dude, sister, news flash: it started. When you get to Heaven? That's stupid. When the kids are grown? That's stupid, hateful, and irresponsible (see below).
It matters to your children. This is the saddest part. We all say we think it's the saddest part -- even people who tolerate and even cultivate marriage-harming sin in themselves pay lip-service to caring about The Children, while their lives show they care a whole lot more about unrepentantly indulging their flesh in this or that way. But kids see, they notice, they take note, they know at some level, and they are harmed. Not merely discouraged, distracted; harmed. Home should be a safe, healthy, Christward place for them. They get their ideas about marriage by watching yours.
It matters to your church. Maybe not right away, because it is possible to put on a good face for an hour or two a week. But you can't be giving yourself to godly, Spirit-led worship and service with a bunch of relative strangers if you're not obeying the great and the second commandment (see numbers 1 and 2, above) at home, can you? Does that make any kind of sense? Seriously. Wake up.
I plan to say a lot more with a lot more specifics. If you want a lot of Biblical study and content and counsel, you can get a head start right here, as perhaps some readers could attest.