Wilson Fisk said: "In order to build a case for a interpretation of a particular text you need to find evidence with which to support your case."
Not necessarily. To write a fanfic story and still keep it within the boundaries of canon, all you have to do is make sure it doesn't contradict anything we see on screen. We are not looking to prove that they are homosexual or bisexual just that that possibility is not excluded. If not, then it is fair game for fanfic that stays within the boundaries of canon.
Some people however have made interpretations of various scenes between Avon and Blake that suggest a possible sexual or potentially sexual relationship. This in their opinion adds strength to their stories. Just because your interpretation may be different, doesn't mean yours is right and theirs is wrong. Both can be viable interpretations of a scene.
As an aside, there is no rule that fanfic has to stay within canon - there have been some excellent stories that have deliberately not done so, following "what if..." situations. However, some like to keep their stories within canon and therefore look for sometimes very slight clues to help them. There is nothing wrong with this.
-- cheers Steve Rogerson http://homepages.poptel.org.uk/steve.rogerson
Redemption: The Blake's 7 and Babylon 5 convention 21-23 February 2003, Ashford, Kent http://www.smof.com/redemption