If it’s big enough, you can’t even maintain an erection because it requires too much blood. Or wear several types of pants or underwear, or sit comfortably in certain kinds of seats, or sometimes even get the simple feeling that you’re done peeing when you are. But “lol I hurt women with my massive 50-inch pecker” gets more upvotes.
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.
The other big problem is that if it’s big enough, it may be uncomfortable or down right painful for any partner.
Again, that’s usually more of a girth than a length problem, though too much length can also result in pain. (Bumping into the cervix is, for most women, a rather painful experience.)