Why is it that some religious brothers become agitated on a person who marries a second wife despite being able financially and morally, for whomever wants this marriage?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

It is not possible for a truly religious people to object to what Allaah has legislated, as being religious requires a person to fully abide by Allaah's Laws and to submit to Him. This is one of the requirements of faith. Allaah Says (what means): {But no, by your Lord, they will not [truly] believe until they make you, [O Muhammad ], judge concerning that over which they dispute among themselves and then find within themselves no discomfort from what you have judged and submit in [full, willing] submission.}[Quran 4:65]. Allaah further Says (what means): {It is not for a believing man or a believing woman, when Allaah and His Messenger have decided a matter, that they should [thereafter] have any choice about their affair.}[Quran 33:36] Anyway, those religious people who object to what is mentioned in the question, they do not object the principle, but object due to the injustice which some people who marry a second wife practice, as they do not fear Allaah in this matter.  

Allaah Knows best.