Well it's their choice; I myself find it a bit immature. Being 100% straight is kinda what you are before you know better and realise that's a really restricting state of being. You're shutting off half your options.
Don't want to sound prejudiced (straight people can't help the way they are) but it's their loss.
I don't think its completely human. Being atracted solely to the same sex is typical animal behaviour. In a way it could be considered a mental illness in that a person has not completely evolved from their animal roots.
Oh don't worry, I will. As soon as the eejit who started the 'Do you approve of homosexuality?' thread gets one I'll be first in line for one of these 'grips'.