Is it okay for a 15 teen year old boy to be gay?

Yes, they can. 14 is beyond the average initial age of the onset of puberty in males.

And, regardless of this, there has been well-documented scientific evidence pointing to a genetic link to sexual preference based on the reactivity of the vomeronasal organ to gender-specific pheromones in human beings. Therefore, a person can be born homosexual versus heterosexual, and they can have this instinctual gender identity long before they actually reach puberty.