I think its really important to remember that a relationship in its entirity takes work.
At the beginning of a relationship or if people marry, they commit to this work, to making the relationship work
But sometimes, along the way, especially in the sleep deprived haze of a young family, both partners forget about this committment and expect things just to "be" good without work from either side.
Sex is part of a relationship and I think it needs work. This includes kind touch, doing housework (all part of foreplay in my opinion) and trying to work it into busy, sleep deprived lives.
And it probably means not getting into the "who has time/energy/desire for sex" talk with people, which is a great way of normalising not putting in the effort.

I also think having sex is the best way to increase libido. Just as for most babies sleep begets more sleep, for most couples sex (especially good sex) begets more sex.

The committment has to go both ways. The person (not always male) in the relationship with a higher libido, should commit to working on other areas, which are often doing more around the house, giving their partner naps etc.

Also as a relationship ages, sex often becomes a little old, so sometimes injecting something new and exciting is a great way to make it interesting again.

Of course, these issues are always easy to talk/think about and often less easy to act on.