Yes, you can drink your Hydro Flask without the straw.
The Hydro Flask comes with two well-designed straws and is easy to use. However, if you prefer not to use them or lose one of the two that came with your Hydro Flask, it’s easy enough to remove the remaining one or both.
You can still drink cold beverages by tipping the flask and letting gravity do its work (though this may not be as easy for those who have had major dental work).