Being out in the sun isn't the problem, it's being in the heat. Dogs get hot just like humans do and need a place where they can get out of the heat. You can leave them outside you need to make sure they have plenty of water and shade or they can have a heat stroke. You have to realize that having fur all over their bodies makes the heat much worse for them than humans
It's bad for any animal (including humans) to be in the sun for too long. The dog could get sunburned, dehydrated, or have heat stroke just to name a few. Don't leave animals in the sun, and while I'm on this subject.. don't leave animals in the car with the windows up in summer!
Because a Boxer is a brachycephalic dogs which means having a short broad, pushed in muzzle...the Boxer is not a high heat tolerance dog because it can over heart easily and the dog is suseptible to breathing problems and heat stroke in high temperatures.
they could get skin cancer, dehydrated, dizzy, catch diahrea, get attacked by stroke, be higher in rish of heart attack, possibly get asthma. Make sure they drink plenty of water!!!!
boxers are considered one of the bigger braxycphalic (or however you spell it)breeds so they can over heat fairly easily. and with such short fur they can get sunburned easily too.
Don't leave them in the car with the windows down either. If you're not willing to sit in the heat then don't make them do it either.