You don't have to; either sounds natural. In speaking fluently, if they're apples that we've never talked about before, then i would say "The men eat apples." But if you and I were just talking about picking the apples that they are eating now, i would probably say "The men are eating the apples." Does that make sense?
you can definitely say "The men are eating the apples" in English if you are talking about a specific batch of apples. If you are talking about the specific apples you bought you'd say "The men are eating the apples" but if you are just talking about in general or an observation then you can just say "the men are eating apples"
Sometimes if apples are mentioned before the statement, then 'the apples' can be used. If apples are being mentioned for the first time then no 'the' appears. For example, They went on a picnic. In the basket there were apples, milk and some cheese. The men started first by eating the apples. Hope this helps!