I have to wear sometimes more tight underwear because my flaccid hang not to mention testicle hang has become so long at times that its annoying when it keeps slapping my leg and coming out of the underpants, so I wear more compact underwear to hold it all in. I see no problems with it affecting gains, obviously never wear these so tight as its not healthy but a good hold is always nice. It feels much more comfortable for me versus wearing baggy briefs as my cock and balls always hang through.