Is it normal to be 121 pounds for a guy whose height is 5'9 ft

I think you're hot come 'ere but seriously though don't sweat it weight is a very tricky thing perhaps you'll hit a certain age and you'll gain some without even changing anything in terms of diet and exercise. If it bothers you that much as suggested bulk up at the gym but if it doesn't bother you stay the way you some girls love a tall skinny guy.