Done it.
Sadly, it would take a very brave footballer to come out. More likely it will be someone near the end of the career who has boozed / gambled / just wasted all his money and is persuaded to sell his story.... a bit like Justin Fashanu.
Like Dave Cooper, I think fans would get used to it fairly quickly. Fans have evolved since the days of throwing bananas at black players, to such an extent that England fans now take a deserved pride in defending their multi-racial team / support against the attacks of countries where such attitudes are still acceptable. I hope British fans in general would, likewise, take pride in being mature enough not to abuse gay players.
God help the first Iranian / Nigerian / Jamaican to come out though.