On the subject of there being no openly gay premier league football players, or football league players in general (I understand that there is actually only one semi-professional non-league player who is openly gay), the starting point for that is I would say is that the football industry is about as homophobic as you can get. To give some evidence for that assertion, consider the case of Graeme Le Saux, one of the top English footballers in the 1990s. Le Saux suffered from allegations about being gay, despite not actually being gay, from other players throughout his career. This news story from 1999 gives an example:
Le Saux's main 'problem' seemed to be that he just wasn't seen as 'one of the lads' - he was university-educated, a Guardian reader and was interested in things like antiques. A few weeks ago, I noticed he was the 'Dictionary Corner' guest on 'Countdown', probably the only footballer to have appeared on that programme.
To get into the football industry, you don't just turn up for an interview like in other jobs, there is something like a five year or more 'induction period'. You would have to start off as a talented schoolboy player, then you get signed up by a football club and rise up through their youth system. A gay teenager would have to put up with a homophobic atmosphere for several years, where fellow players would be suspicious of someone who didn't appear to be 'one of the lads'. I would expect the number of gay footballers to be significantly lower than the average percentage for gays in society, and any that do make it would be very good at 'acting straight'.
The football industry might underperform on the LGBT quota, but on the other hand, it has to be said that they do massively overperform when it comes to the BAME quota. According to this link, on the opening day of the 2012-13 premier league season, black players represented 32.2% of the total, and of the English players, 34.5% of the total. So about a third of the players in the premier league are black. In this particular case, the BAME representation is dominated by B with hardly any contribution from AME.