Can i fire my personal trainer for sleeping with my sister AND my gf?

Are you really serious? You honestly are not aware that you can stop your training sessions? Listen: yes, you can stop going to your personal trainer because he was getting too personal.

You may not be able to get a refund on your training contract, however, if you still want to train, go to the management and demand another trainer to replace lover boy.

You can fire for almost any reason.

You can fire him for any reason you like. The colour of his socks if you want.

if you don't have an employment contract with him, you can fire him for no reason at all.

That or ask him for some pointers.

it could be cause for ethical grounds. sure. but, as others have said, most employers hire employees on a at will basis. meaning they can let employees go for any reason, at any time, even without needing to give one.

No

Depends on the terms of any contracts you may have with the trainer. If it is an "at will" employment relationship you do not need cause to terminate their employment.

Sure, go ahead. Unless you have a contract, you can fire someone for any reason you want or no reason at all.

You don't need a reason to fire your personal trainer.