Even though Mark makes several points about being proud of being gay, as well as comparing himself to Nigella Lawson and saying that Tom Daley looks better naked than he does (not that surprising), the issue here is not a gay issue at all.
I quote: "The Sunday Express's Head of HR … believes that the existence of any such images could bring the company into disrepute. My sexuality is public knowledge and by revealing this information freely as I'm doing now, it is clear that I do not agree." The Sunday Express's concerns are apparently only over the content of the images, not his sexuality as he tires to persuade us.
I have always worked for companies that are very proud and protective over their image. I expect I would be sacked if there turned out to be naked images of me that could be connected back to the company.
For those wanting to see other occasions when this sort of thing has happened...
These are not direct comparisons as Mark's images are fully-naked images, where as in these cases people were fired for less/