I grew up in the Palm Springs area (summers regularly in the 100-125F range), and played soccer my entire life, through the summer. It's absolutely possible to do it, although it is not fun. The issue is that you have to hydrate completely differently, and to be fair, our substitution rules were different (unlimited subs, and you could be subbed back in), so players who have never played in that environment are at risk. Also, it gave us an extreme home field advantage whenever clubs from the California coast came into town to play.