Is it unlawful for me to resist my desire for a second wife, even though marriage is permissible, for fear of my relationship with my first wife deteriorating?

Polygyny is permissible on condition of justice. Whoever fears not being just, must content himself with one wife. Resisting the desire to marry another is not forbidden; rather, a person may be rewarded for abandoning a permissible act out of patience and to preserve the wife's feelings. Abandoning a permissible act while believing it to be permissible does not mean prohibiting what Allah has made lawful.