Football

Eventually yes they would. but it would take a fuck of a long time before they became unmateable with humans as we know them.
It's possible that if Neanderthals were alive today, we could successfully mate with them and that's going back hundreds of thousands of years. Though they are in the same genus.
West Ham fans. Hmm it's a different ball game. Similar to those creatures in The Descent. I wouldn't fancy my chances 'hanging out the back of' one of them

Of course. We're missing the trick. They might ACTUALLY be a different species.
Occam's razor and all that.

Perhaps we weren't that bothered with keeping decent records of who lived where, way back when, and well. Those that lived out East are a different species and became WHU fans. NOT that WHU fans became a different species.