It doesn't "matter" at all in the scheme of the universe (but what does?), it's just idle fun to think/talk about. If you're even mildly into the celeb it's fun to talk about. And it's like running a calculation and sussing out a likely conclusion based on any number of factors - stats (birth place/time), ethnicity, what they've said (like a couple of the "One Direction" dudes openly talking about it in an interview!), pics, etc. Of course it's alluring since it's the main single body part we can't see! Plus, most guys are into their own dicks and given the userbase here, other guys' - celebs just unify that vs., say, the crush next door or down the street which most other people are never going to see.
It's kind of "celebrity 101" topic of wondering/conversation for male celebs, incredibly common - as you say with the "every single forum". If you're not into it, just feel free to skip past the posts in question or unwatch the thread and spend the time where you want to. Never quite understood the psychology of people who enter a thread to wag a finger and say "Celeb is ugly" or "Stop doing [whatever it is they don't like]". I know LPSG posters and staff work hard to try to keep threads positive and upbeat for people who want to contribute all kinds of stuff related to the celeb, dick speculation included!
