The pansexual pride flag started showing up on the internet around 2010. As the designer is not known, we do not know their thoughts on the color choices. However, the community has embraced the flag that literally means "all sexual" and has chosen to interpret the color meanings to be -
Pink: Attraction to the feminine
Yellow: Attraction to other genders and sexualities outside of the traditional binary
Blue: Attraction to the masculine
If you know any further information on the designer, we would love to give credit where credit is due.