How do BP and TRP explain the (drastic) decrease of sex frequency in marriage? What would both sides do to fix it?

Note : Let's imagine a scenario without children and avoid the "don't get married" answer, as it takes us nowhere.

It does seem that marriage is the death of all that fun sex that preceded it. It's a generalization that seems to be accurate for an important amount of couples. Historically, it has been blamed on being busy with "life" and women having "lower libido". That women, once they got their life settled, just relaxed when it came to sex and so on.

However, the concept of "gaming while in a relationship" has kinda shifted my idea of it. If in the past I subscribed to the theory above, now I wonder if the main aspect is how the man lets himself go, gets comfortable and the woman naturally starts losing attraction, which starts a bad cycle.

So, how do both sides see it actually?