If there are not equal, the survey sample is not random and the survey error tends to be bigger. You may use a stratified sampling method. In most survey proposals, you can see people put a lot of energy to discuss sampling issue. It makes sampling issue much more important than it really is. Because sampling error is calculable.
People can demonstrate their professionalism by calculating such sampling errors.
Few people talk about that in any survey design. People argue that because online survey is self-selected, the sample is not random. Therefore, the result is less reliable. Unfortunately the argument is partly correct. However, although the survey result becomes less reliable because of self-selections, a well designed survey produces the most reliable result we are able to get.