It was just a preference, nothing more. Though he did wax poetic about it and did try to find the sorts of hot drinks people preferred. It was just a game he played with himself.
And, yes, black teas were nearly as bad as coffee, but not quite.
"That's alright. There's a number of people about who don't have some of the things we have here. Where they're from, I mean. A dear friend of mine, River, never had coffee before I made it for her. I think she prefers tea, though. Some really nice herbal blends." Ianto, meanwhile, was excellent at small talk. He was excellent at being whatever he needed to be. It was a hold over from his previous job.