No, sewing cotton fabric with polyester threads, as they are made today, will NOT cut the cotton. It will outlast cotton. Many of the replies here seem to be repeating things that may have been true in the 50's..and even the 60's...but poly thread has changed a lot over the years. It's perfectly fine to use on cotton. If you look at your Coats n Clark thread...it's cotton wrapped polyester...because 100%, 50 wt cotton will not hold up to repeated washings and use.