I believe the science of this comes from the way we have evolved to form romantic connections. It was recounted to me best by a friend of mine who studies human nature, men on an instinctual level are permanently looking for their partners to prove their sexual commitment to them. While women are permanently looking for their partners to prove their nonsexual commitment to them.

So men have a permanent voice in their subconscious that says to their partners, prove your sexual commitment to me i.e. prove you're going to let me pass on my DNA through you and that you are not going to allow another man to impregnate you.

Women have a permanent voice in their head that says to their partners, prove your nonsexual commitment to me i.e. prove that if we have children you're going to stick around.

Whether we want children or not it is likely we predominantly are driven by those subconscious needs. And they tend to never leave us no matter how long a relationship lasts.

That means if a man's partner stops having sex with him or has it seldomly, he will basically feel greatly devalued and unwanted because the instinctual need of permanent proof of his partners commitment to him is not being met. And if a woman's partner stops spending time with her outside of the bedroom, she will basically feel greatly devalued and unwanted because the instinctual need of permanent proof of her partners commitment to her is not being met.

Of course, as with any theories they should be taken with a pinch of salt, but I have always found it does make a lot of sense. And it perhaps is beneficial for men and women to realise that instinctually we potentially think differently around time spent together having sex and not having sex i.e. we tend to feel validation in different ways.