Those who gamble for fun anyway have their limits. They cant change goal again and again. There will be a moment when greed factor comes into play. No matter how they convince themselves that they gamble for fun and money is just a paper, they will face a moment when they will consider pity to lose such an amount. I believe that those who gamble for fun withdraw their winnings, but compared to other gamblers, they gamble more risky, more reckless, more careless, more spontaneous.
I believe that most people who bet just for fun usually play with very small amounts of money, the kind of money they wouldn't care about losing at all. For example, there are players who bet no more than what they'd spend on a coffee and a pastry in the evening.
I don't think it is that simple. How do you define playing for fun? Is it related only to the amount of money that you bet? No. If you just come to play, with no target of winning and no strategy then you can't have negative emotions about the outcomes. In this case, you are just playing to pass time or to entertain yourself. This can be also done with a bit larger amounts and not only coffee money.
Losing that kind of money isn’t something to regret, because most people can easily afford it. As long as you don’t cross that line, it can be considered entertainment. But if you say you’re playing for fun while raising your stakes, that’s already self-deception.
There is no exact limit of loss by which you can define a difference between gambling for fun and gambling to win. It depends.