Can Muslims Do a Workout in a Mixed Gym?
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QAs-salamu `alaykum. Is it haram to work out at a mixed gym that has males and females if though the intention is just to work out? Also, in the facilities, the men's locker room only has open showers, is it haram to shower in an open area showers?
ANSWER

Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.

In the Name of Allah, Most Gracious, Most Merciful.

All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.

In this fatwa:

You can try to choose the most suitable shift for exercising or choose a place where you have a privacy for working out and that you do not see naked or mixed exercising people.

In responding to your question, Prof. Dr. Monzer Kahf, Professor of Islamic Finance and Economics at Qatar Faculty of Islamic Studies, states:

Exercising in a mixed gym where there are other persons, males or females, who have parts of their bodies uncovered is really problematic.

Can We Wear Shorts in Sports?
Under extreme circumstances where they are few and one can lower his gaze, I cannot claim that it is haram. This is only for persons who live in Western countries environments where it is difficult to find male only places of exercise. Showering naked is not permissible.

Of course, one can avoid this and most of such clubs open for extended hours that one can go at odd times when there are almost no females.