Is intentionally ejaculating merely by thinking of one's wife, without any permissible or forbidden action, permissible or forbidden, and what is its effect on fasting?

Thinking about one's wife to the point of ejaculation is not sinful, unlike thinking about a non-mahram woman. Ejaculation in this case does not invalidate the fast according to the majority of scholars, but it does invalidate it according to the Malikis, who also require expiation if one continues to indulge in such thoughts. Engaging in this for a fasting person is contrary to what is recommended and incompatible with the etiquette of fasting, as it is one of the desires that one is encouraged to abandon while fasting, and because it involves indulgence that is not befitting the wisdom of fasting.