Will it be a bad move for Master's to do summer internship in industry when trying to pursue a PhD program? Currently, I am in second semester of Master's program in Computer Science and Engineering. Even though I am yet undecided, I lean a little bit more towards pursuing PhD program rather than getting into industry.

Many people tell me that research experience is the only factor in getting accepted to PhD. The problem is that I have a summer internship offer from an established industry, and I do not have any professors that will accept me as a summer research intern at this time.

Personally, I do want to try out the internship to see what it is like, possibly helping me decide whether to definitely pursue PhD or look for more industry options. However, as of now, I feel that it will be a bad move in terms of applying for PhD next semester, where I will probably lack research experience that other candidates might have. Meanwhile, I am trying to build research experience through course projects and directed research within academic semesters.

How much adverse impact will it have in PhD application if I were to choose summer internship at the industry?
A person who has had his finger in both the academic and industrial pie, and then tells me that they want to do a PhD, comes across as more credible. You understand what each has to offer and you are more likely to stick with your choice. As for research ability - it has to be quality, not quantity, at this point. The rest surely will come in the course of a good program. 

If the internship is interesting I would take it. Just make sure you learn something while you are there - do more than "just your job".