This is definitely an option, but I do want to point out that sexless marriages are generally not not black and white. Often the person who doesn't want sex with their current partner has a complex reason why -- it has more to do with the relationship than the individual just not wanting sex. For new mothers who are the primary caregivers for the children, it makes sense they wouldn’t want sex because of all the reasons you pointed out. But that doesn’t mean it’s going to create fairness to give the husband a hall pass to start sleeping around, taking even more time away from caretaking responsibilities. That’s undoubtedly going to lead to resentment and even less likelihood of sex returning to the marriage.
Even if children aren’t the reason sex has faded, maybe partner B is just not treating partner A in a way that makes them feel sexy anymore. Or maybe they've simply grown apart in their sexual desires, for example, one person may be discovering kinks their partner isn't into. Most people do want sex -- just not necessarily the way it's offered to them by this one other human.
My opinion: if one person gets a "hall pass," it needs to go both ways, and the couple needs to begin practicing some form of mutual ethical non-monogamy. If they're going to stay together, I also think it's important they investigate and understand why their relationship has become sexless or jealousy and hurt will most certainly flare.