dating' does sound as if they want to emphasize that it's not necessarily a serious or long-term relationship, just one in a series of girlfriends.
To me the main difference is that if two people are just dating, they may not be living together (all week). Once people combine households and do everything together, not just dates on weekends, then we're more likely to say that they're in a relationship, have a partner, are a couple, etc.
None of this probably matters for a cosmetics survey, in which 'dating' could mean either dating one person, or going out on dates with various people. As we discussed, the latter could be covered under 'single' -- but on the other hand, there are also plenty of single people who aren't actively dating, that is, looking for a partner.
Since there's only the one word in English, it would help if there were an option that was equally vague / open in German. But I'm not sure what that would be.