Is refusing to marry a man who proposed to me—despite his persistence and desire—considered an injustice to him, especially after performing Salatul-Istikharah and receiving news that he is a "womanizer," and due to my illness which prevents me from forming a normal family?

If a man proposes to a woman, inquiry should be made about his character from trustworthy individuals. If he is a man of faith and good character, the woman should perform Istikhara (prayer for guidance) regarding him.

The essence of Istikhara is to entrust the matter to Allah, and the problems you mentioned may not necessarily be a sign from the Istikhara.

You are not obligated to accept him, but if it becomes clear that he is a man of faith and good character, then agree. There is no blame upon you for marrying him despite your illness, as long as you have informed him and he has consented to it. It is not permissible to describe a Muslim with a defect without clear evidence.