Briefs are more comfortable for exercise than boxers. Boxers let your boys get caught and pinched on bike seats and what not. Briefs tuck them neatly out of the way.
If you want your spouse to get pregnant, wear boxers. Heat is not good for sperm production.
Years ago I read a study about men who worked in pizza kitchens. Most of them had a very low sperm count because of the heat. They did a study to try to get them to wear a refrigerated device to keep their sperm cool. All the volunteers dropped out of the study within 3 days.