Under Islamic marital jurisprudence, Muslim men are allowed to practice polygyny, that is, they can have more than one wife at the same time, up to a total of four. -- Polygyny in Islam, Wikipedia
Among my group of Muslim friends, I'm not aware of any men who have more than one wife, and any women who have a co-wife. Most people seem uninterested in the idea, and the married men I know seem to treasure their wives. I'm not sure how this generalizes to Islam, and the Muslim population.
Question: Is marrying a second wife makruh (disliked) even if the husband can treat his wives equally?
The Wikipedia page has a section Opinions of classical Islamic scholars on polygamy which has a repeated theme that having a second (or multiple) wives is not encouraged. The two sources it provides are an IslamWeb fatwa and a link to PolygamyInIslam.com (which looks like a dead link).
The IslamWeb fatwa argues (a) a man should avoid it to avoid the possibility of being unjust to his wives and thereby sinning, and (b) if the husband really wants a second wife (particularly for reasons of chastity) and can be just to them, then it's fine.
Perhaps the content of the IslamWeb fatwa (or some other fatwa) implies the answer to this question is "yes", but that's not clear to me. Or maybe the answer is more along the lines of "it's nuanced, and doesn't have a simple makruh/not makruh classification in general".
site:islamqa.org second wife makruh,
site:islamweb.net second wife makruh,
site:islamqa.info second wife makruh doesn't give any immediate answers, which makes me think it's not considered makruh.