Just for perspective: I am a man in my mid-40s and for as long as i can remember i had this question… is it normal that i can shut my nose close just with some muscles?
I always wondered why some people hold their nose shut with their fingers while jumping into a pool or when exposed to some horrible smell… i never had this need. Changing my sons diapers? No problem! Cleaning the latrines while in the army? Just a breeze!
Ehm… yeah… just thought about this…
Yes, closing your sinuses is a natural reflex response for humans, and people have greater or lesser at will control over it.
The nose holding for swimming is more about how strong that sinus closure is and endurance. People with larger sinus openings have a more difficult time keeping them closed and resisting pressure like water entering from jumping into a pool. Also some people have a hard time keeping them closed for any prolonged period.
In other words, you just have totally ripped sinuses breh.