This heat always raises the same question in bikers. Safety or comfort? Having ridden across the Italian plains a few years back in the height of summer, I'm familiar with riding in searing heat, but that was a different experience to commuting.
The ride across Italy was a much simpler affair. I put the kit on in the morning, rode the bike, got really hot, drank lots of fluids when filling up, got to my hotel, took off my kit, showered, had dinner and went to bed.
Commuting is a totally different beast in the heat, as you're roasting by the time you get to work and then, well for me at least, really struggle to cool down. These problems lead many to ride in less protective kit, sometimes shorts and a T-shirt and no gloves, which subsequently splits the community right down the middle.
You have the 'sweat is easier to spill than blood' camp on one side and the 'its feels so refreshing' on the other. I fall firmly in the first camp and really don't like riding any real distance in anything less than decent kit. If asked my opinion on the matter, I usually return the question and ask them if they remember tripping as a kid, putting their hands out and grazing their hands and arms on the concrete? The response if usually 'yes' to which I reply, now imagine that at 30mph and weighing three times as much, often with a motorbike on your back. It certainly gets people thinking.
Despite my feelings on the matter, if somebody wants to ride without greater levels of protection, then who am I to tell them otherwise? The legal requirement is just a helmet, nothing more, but I do draw the line when I see pillions without any decent kit on. I remember in London seeing a sportsbike rider, in full power ranger mode wearing a lovely set of Dianese leathers, boots, the whole deal, with his girlfriend on the back in flipflops, a little dress, t-shirt and helmet. The contrast was striking.
As a pillion you have all the risk and none of the control. Non-bikers think we're mad, but bikers know for a fact the mad ones are the pillions. If you want to increase the risk to your life, then thats your choice, but don't inflict it onto your passenger.