No, you're not crazy for going in June. You'll have to be in summer mode, though, climb early, take a swim and a siesta from 1 to 4, then climb 'til dark at 9 pm.
The only problem you might have is finding someone to climb with.
As far as beach resorts, don't bother with anything in the Gulf or you will be disappointed--better go to the Carribean, say Tulum. If you want to see the best Mayan ruins, and a very cool all-around place, go to Palenque in Chiapas.