I think the following site are dangerous to link to: Adult, casino, pharmacy, credit card(?), anything else?
those sites are inherently dangerous to link to; any site that is using questionable techniques is dangerous to link to. but if you are an adult site, linking to other adult sites that are honest in their SEO is not a bad thing.
I don't think it dangerous if you are relevant to there business. Just be sure to take a good look at there site to ensure no funny business is going on? I know of lots of legit websites in all of those categories.
Whilst there might be a higher than average number of dishonest webmasters in these industries it is not right to make generalisations like this. You have to give it to the porn industry for being at the forefront of some legitimate innovations such as being able to process small payments profitably, just to name one, which can be used in many other industries. As with any link you place on your site, one of the primary reasons for putting it there should be that it might be useful to your visitors. For example, if you have a site about blackjack strategy why not put a link to an online casino. Many of them are just as legitimate as land based casinos and no-one seems to have a problem including them on sites such as travel based ones.
Most people dont want to post links to those kinds of sites b/c a lot of them are a)spammy and b) not reated to their own sites. I dont blame you for not wanting to link to them. Nothing against the decent folk who run the decent sites, just better safe than sorry...
regarding pure ranking in the SERPS, none of the sites you mentioned is "dangerous" to link to (unless of course, an individual site is banned by Google). Bulk linking to adult sites with the appropriate anchor texts can lead to your site being recognized as an adult site by various search engines. This leads to your site being filtered when searching in "safe mode" (non-adult mode). This does not apply to credit card, real estate, casino and pharmacy sites of course.