Does exercising increase the rate at which UV rays are absorbed and cause affects?
The only pathways in which I would see more sensitivity to UV light would be


If you were drenched in sweat and somehow the sweat focused the light/UV.   This is highly unlikely as I don't believe any studies have been done to show that a water based solution causes a more intense sunburn.


(Here is a further explanation by another poster on wetness and sunburn https://physics.stackexchange.com/questions/71263/why-does-wet-skin-sunburn-faster )


The increased dilation of blood vessels results in a more red thus darker and more absorbent skin tone leading to more of a sunburn


Both of these are highly unlikely of increasing the sunburn, so in my opinion exercising shouldn't increase UV absorption or make a sunburn worse. Hope that helps.