Want to understand the decline of sexual intimacy in marriage?
The most common human female evolutionary mating strategy is this: use concealed ovulation to secure a relationship (safety and resources); use fecundity to secure her own genetic lineage (not yours, HERS); then slowly reducee the frequency of sexual intimacy to avoid additional pregnancy and possible disease that would take away from her existing lineage, increasing sexual access only as necessary to maintain her own access to safety and resources while she ensures the survival of her own genetical lineage.
In short, if you've given her children - or if she already has children by another man - the only reason she has sex with you is compulsion towards her own pleasure, or to keep the safety and resources flowing.
None of this is conscious. But this is why access to intimacy fades over time in many stable relationships. Once a woman has children, absent an innate compulsion towards sexual intimacy, if she has a lock on safety, resources, and caloric surplus, she's no longer "in the mood" unless she needs safety or calories to survive.
Stability, for all too many women, is the death of desire.
She'll perceive this as boredom and the sense that you're not good enough for her. She'll complain that you don't do enough housework, even if you do. She'll complain that you're not enough of a parent even if you are. She'll feel that you're not off of a man even if you are out of her league.
And this is why many women destroy their own relationships. They are never with their first choice even when they are with their first choice, because once they have what they want from their first choice, it's simply not good enough, in some ambiguous way they can't define, that precipitates on whatever momentary irritant is in the relationship - but the real problem is, she's just too damn stable and needs some adrenaline.
You can't be the source of that adrenaline. You can't deny her your resources, or you're an abuser. You can't make her afraid, or you're an abuser. Any threat that comes from inside the relationship is going to have you labeled as an abuser. The threat needs to come from outside the relationship, so she will turn to you - but those threats rarely or never come today.
So you compensate by buying her nice things, showing her your respect, taking her out for nice dinners, washing the dishes, looking after the kids, trying to find any way to make her happy - but it's an impossible mission, as long as she has caloric surplus and safety.
What she really needs is to feel threatened and to see you as a solution to the problem - she needs to see you act like a man and take care of business... but there is no business to take care of, in part because you've helped create a stable life for her.
Eventually, she starts thinking about other men. Why? Because she's too stable. She needs a roller coaster ride to feel alive. You aren't that ride - you were when the relationship was new, but now she's too stable. She wants a roller coaster ride, and she can't admit it or can't articulate it because on the surface of her mind, she WANTS to be safe. But deep underneath, she feels that it's somehow wrong.
Sexual intimacy inside monogamy, to women, was a solution to a problem of external existential threat. Absent that external existential threat, monogamy is a struggle for many women. They want it, but at the same time, too much safety and surplus bores them horribly.
There's nothing you can do about it without being labeled an abuser - except spend more and more money and energy on roller coaster rides. The solution ultimately must come from inside herself as a function of her own character - just as men must manage their own latent desire for variety.
Monogamy is tough. The incredible safety and surplus of our society has made it harder. Absent existential threat, monogamy is too tough for many women with high optionality and safety.