is it because you want to have insurance that they will stay with you
is it so you don't have to share with anyone
is it because you're afraid if they get to go around they'll go away
the "exclusivity" you want is not made out of honest wish to be with just one, it is made out of fear of losing the other person or jealousy, simple as
you paint it as a nice thing by glorifying it but in reality it's all just you feeling insecure about the other person and being afraid of losing them, for a good reason or not i can't say but there it is
if you had 110% faith in the person that they would not go catching feels for others and wouldn't leave you would you still want exclusivity, if yes then why?
see humans are not as pretty as we dress ourselves up as, we are actually pretty nasty people who just like to play dress up in roses and rainbows