From what I've heard, a big issue is condoms. Length isn't an issue, girth is. Too large or too small and it rolls up and/or is more likely to tear. Like a too small latex glove, except a dick there's nothing to stop the condom rolling up. Cleaning a toilet and the glove tears? Shit happens. Condom tears? Baby or STD happens. Not funny.
But from what I can tell, a lot of people who post in forums like that are roleplaying having a large dick or gay guys who want to creep on guys with large dicks. People are weird.