- prát = to do laundry, to wash clothes
- prát se (reflexive) = to fight
- Kluci často perou = Boys often do laundry
- Kluci se často perou = Boys often fight
Fun fact: it's the same in Polish: prać = to do laundry; prać się = to fight, although the second one is very colloquial.
I'm guessing that this dates back to the time when one beat laundry to wash it.
There is little connection between perfective/imperfective verbs in Czech and simple/progressive tense in English. If anything, the imperfective aspect tends to be expressed by simple tenses, and the perfective aspect sometimes by progressive tenses - i.e. the other way around from what you're suggesting.
The plain "Kluci se perou" (imperfective) could of course mean both "The boys are fighting" and "Boys fight".
That aside, I don't think English likes the combination of "often" and a progressive tense, they don't match well. https://books.google.com/ngrams/graph?content=often+do%2Care+often+doing&year_start=1800&corpus=26&smoothing=3&year_end=2019&direct_url=t1%3B%2Coften%20do%3B%2Cc0%3B.t1%3B%2Care%20often%20doing%3B%2Cc0