MDMA has been synthesized by this point (in Germany, no less) but people don't really know what it does yet. Is there possibly a way that someone experiments with it, realizes what it does, and somehow gives it to both Hitler and Chamberlain when they meet? Perhaps someone who accidentally did something gay while experimenting with MDMA and wants to bring down Hitler.
I know it doesn't "turn people gay", but touching people produces lots of feel good hormones. What if an extended handshake in which you use both hands lasts too long, one of them interprets it in a certain way and reacts in an ambiguous way, etc etc. It's obviously an extremely long shot, but improbable things have happened before in history.
I don't think Hitler and Chamberlain could become lovers, but imagine if in one particular moment something sexual happened between them.