What is power in a regards to a relationship?
If power means the ability/capacity to do something or act in a particular way and the physical strength or force exerted.
Then, the power of being the provider, protector or supporter via the ability of being the primary income earner and from physical superiority related to musculature strength, robustness or toughness is undoubtedly possessed by the man.
Moreover, these roles are compatible with the masculine traits that men possess which also play an important criteria in sexual selection by the females, commonly known as female hypergamy.
On the other hand, power related to domestic affairs, bringing up the children, transforming a house into a home or in preparing a loving meal for the family, then this power belongs in the women's domain. These roles are congruent with the feminine features like empathy or compassion which makes a woman both sexually attractive and having great potential to be an incredible mother are important for men in mate selection.
In essence, men make great providers to the household while women make great carers for their family aka primary income earners and primary carers. Hence, frequently in mate selection, men put emphasis on femininity and physical appearance but women give higher priority in the potential ability of mates to provide resources including protection.
Therefore, this "arrangement" works out well for both men and women, as it utilizes each of their, very natural but distinct gender traits for good use to maximize the benefits resulting in optimum effect from their partnership. It is pretty much a win win situation
Similarly, power in regards to the capacity/ability to direct or influence the behaviour of others or the course of events it would involve the man too, because the man is often seen as the authoritative parent in a family. Likewise at work, men are required to show leadership and maintain "law and order" in an organization. Once again these roles are in keeping with a man's masculine traits.
Nonetheless, when it comes to sex, the situation can be quite tricky and very oppositional most if not all of the time. Why?
This because the sexual natures of men and women are very different in regards to sexual behaviours, motives, preferences, tendencies, attitudes and feelings.
These differences in sexual nature between the sexes cannot be changed as the reproductive rules have been set in opposition millions and millions of years ago.
Moreover, among humans these sex differences were further reinforced during our own human evolutionary history whereby the sexual desires and inclinations that were adaptive for men obstructed reproduction for women, while those that were adaptive for women obstructed reproduction for men.
These differing sexual natures between the sexes are partially obscured by moral rules, social norms and most importantly the compromises inherent in relations between the sexes itself.
However, strictly monogamous sexual relationship practiced today is solely beneficial for women, notably more than men. Thus, the power in our society involving sexual relationship is usually held by the women. Now, if there are good compromises and fair understanding is achieved between the couple, then it would be ideal. But often, women can feel very empowered from such a situation with potential to cause disharmony either by deliberately withholding sex from the man for any reason whatsoever, or using it as a bargaining tool, a weapon, as emotional blackmail, some sort of a ‘punishment’ method or even just to feel ‘in-control’ of the man. Such toxic behaviour by women is considered to be very frustrating by many men and rest assured, not only such relationships are doomed to fail badly, but there is complete justification for men in these situations to seek sex with someone else if strongly desired especially if there is complete denial of sex by women.
Furthermore, unlike females, sex plays a very crucial role for men in a relationship, as strong sexual impulse and intense libidinal energy form a very central part of male sexuality.
In addition, sexual promiscuity, sexual novelty, variety and quite spontaneous sexual desire are vital features of male sexual nature.
Therefore, in my view, for complete fairness and to sustain harmonious healthy relationship among long term committed heterosexual couple, polygyny or sexual promiscuity for men should be a crucial aspect to be considered and must not be deemed "taboo" in a relationship, provided it is solely a physical sexual relationship without any emotional involvement.
Although most sexual relationship work with compromises, having somewhat gynocentric sexual relationship whereby "males could be produced who would want only the kinds of sexual interactions that women want" can be very disastrous and have many unwanted adverse effect in the long run. On the whole there are many reasons that this can benefit and sustain long term relationships.