Adam is a great believer in science. If something is scientifically proven, it's a fact, and Adam doesn't waste his time arguing with facts. Tom is as straight as any man can possibly be, Adam doesn't like it, but he knows he has no choice but to accept it.
When Tom approaches Adam at the office Christmas party, Adam's not going to let himself be fooled into thinking that Tom is flirting with him just because the other man is friendly, or because he doesn't seem to understand the whole person space thing.
Facts are facts, until they suddenly change, in the blink of an eye.