I figured this was just due to Russell T. Davies bringing his long-term acquaintances along, some of whom are exceptionally "out" in the same way that he is. But long-time fan Tat Wood devotes a section to this phenomenon in his terrific (and very funny) series "About Time."
The section is called "What's All This Stuff About Anoraks?" and is on page 167 of volume 6 (accompanying the writeup for a story starring Kate O'Mara, appropriately). The section is a mini-essay about British Doctor Who fandom in the '80s. After explaining the stereotype of the British Doctor Who fan (geeky, cynical boys who wear anoraks) versus the American counterpart (obsessive and slavish young men who wear Doctor Who scarves), he leads into the relevant stuff with the sentence "...but the real statistic anomaly was how gay it all was."
Perhaps spending so much time in an environment where people already knew the most embarrassing thing about you made it easier to make the second-most awkward admission of your life. Perhaps some people found that a hero who never quite belonged, and had other priorities than getting the girl and looking cool, helped them through difficult school years. Or perhaps it was the fact that the Nathan-Turner years were the most overtly homosexual mainstream TV ever transmitted at that point... a significant percentage of fandom in the UK -- and a very high proportion of the high-profile fans and future spin-off creators -- were gay or bi.Wood doesn't try to explain it much beyond this, but I'm sure he has a point.
Incidentally, it never seemed to me that classic "Doctor Who" was particulary gay, but then I stopped watching shortly into JNT's producership...going back now and seeing those stories for the first time I can only say...wow. Total camp and a bit mortifying.