I think there's a difference between thinking/wondering about when you'll meet that special someone in your life to share the rest of your days with, vs. feeling ANXIOUS about when you will meet someone, or why you're single, or finding someone, etc.
I think the latter is what is detrimental. I don't think that wondering or even thinking about relationships or when you'll find "the one" is necessarily wrong/damaging. I think it's the fear/anxiety associated with that which is not good.
You can think about it, but just don't feel bad or anxious about it. In the meantime, focus more on YOURSELF, improving yourself, being the best YOU that you can be, and trying new things, meeting new people, etc.
I found that when I started focusing more on MYSELF this year, not only did I start to feel less anxious about when I would find "the one", but I also felt happier.
I still desire a bf and want to be married (tremendously!), but I have given up 'the search' and I'm learning to just accept whatever life throws at me and be receptive. I'm not just going to rest on my laurels (you'll never find someone that way lol), but I'm going to go out, do things, meet new people, make new friends, but in the meantime I'm also not going to beat myself up for being single at the current moment, or worry about it.
Haven't you always wondered why the people who say they don't want to be married usually end up getting married or proposed to lol?