The other thing I think a lot of people overlook is the millennium puzzle. It gives him this crazy luck boost. His deck might be filled with hyper-situational one-of's, but when you can magically draw the exact card you need at any time, all those situational singletons show up precisely when he needs them.

OP is correct that the scene wouldn't have aged well, but Marty does actually raise a legitimate point. Not about it making him gay (spoiler alert: that's impossible; you are who you are), but in that this really could cause some serious psychological damage. Especially considering that the original plan was for him to be overly aggressive, thus justifying George's intervention in the situation.