Is it a sin for a man to wear a bra if he has gynecomastia?
The reason women wear bras is to support their breasts and to make their shape more distinctly womanish. A man is to appear as a man and not a woman. “A woman shall not wear man’s clothing, nor shall a man put on a woman’s clothing; for whoever does these things is an abomination to the LORD your God” (Deuteronomy 22:5). God made you a male and you give glory to God by accepting your role.
Thus, men dealing with gynecomastia usually wear a tight t-shirt or tank top to make their breasts less noticeable. A bra would make the breasts more noticeable. Since bras are distinctly an item of women’s clothing, they would be inappropriate for men to wear.