Give the Groomsmen a Break
It hardly seems fair that the women in your wedding party are going to be outfitted in lightweight, strapless gowns while the groomsmen will be sweating it out in tuxes. Consider relaxing the wardrobe requirements for your groomsmen if your venue is casual enough to allow a more chilled-out look.
I'm willing to bet that you'd balk if someone told you to don dark pants and a jacket with a bow tie in near-100 degree temperatures! At the very least, don't be offended if the groomsmen ditch their jackets and ties once the photos have been snapped.