It's hard to come up with a good counter-argument to "slavery is bad". Even women's suffrage and Prohibition didn't require lying.

That's bullshit. More precisely, it is quite possible that you don't consider any of the counter-arguments to be good. But you should not generalize it for everyone. A "good argument" is a 2-place word; it means that a given person accepts the premises of the argument and its style of reasoning. Also, there is a lot of hindsight bias and social pressure here: we already know which side has historically won and which is associated with losers; but before that happened, people probably evaluated the quality of the arguments differently.

I could start playing Devil's Advocate and give examples of specific arguments that would seem good to some people, but I am not sure the readers (and our stalkers at RationalWiki) would focus on the meta-argument of "it is possible to make good arguments for X" instead of taking the arguments as literally my true opinions (plus opinions of everyone who upvoted this comment, plus opinions of everyone who didn't throw a tantrum and publicly leave LW after seeing me publish this comment there).

"There are no good arguments for X" is simply how having a successful social taboo against X feels from inside.

For example, many debates with real-life feminists about women's suffrage assume that men had universal voting rights since ever, and women only got them recently. But the truth is that "men's suffrage" (a voting right of every adult man) also only came historically recently. In some countries, both men and women got the universal voting right at the same year. But you wouldn't guess that by listening to debates about women's suffrage in that country.