This comes down to reasonable behaviour on the part of both the employer and the employee. Most workplaces will have a dress code and guidelines on what they consider to be acceptable standards of dress in the workplace.
You should first of all check your employee handbook or contract to see what (if anything) these say about acceptable standards of dress. Most employers will be prepared to be flexible during hot weather and should not consider a smart (but cooler!) alternative to a suit to be a problem. Obviously, standards of dress will very much depend on the job that you do. For example, smarter clothing may be expected where you work in a frontline post, representing your company and meeting clients.
There may also be health and safety or hygiene considerations to bear in mind and these may restrict what you can wear. Even so, your employer should still act reasonably and bear in mind that excessive heat is also a health and safety risk. They should consider all reasonable ways of ventilating the workplace and keeping it cool.
Finally, your employer should act fairly towards both men and women and should not impose very strict restrictions on one sex that they do not apply to the other. This could amount to sex discrimination. If in doubt, ask your workplace union rep if you have one or an advice agency for further guidance on this.