Bamidbor 31 (18) allows the female captive children from the war with Midian to be kept alive. From them there had to be a 'tax' of one out of 500. One half of these was to be given to Elozor Hacohen and one half to the Leviim.
The Ohr Hachaim Hakodosh says that the “keeping alive” refers to conversion to Judaism in order to take them as wives.
החיו לכם. פירוש יגיירו אותם שבזה יקראו חיים כדי שיהיו ראויות להם להנשא לחפץ בהם
Are we to understand that the same was the purpose for Elozor and the Leviim? Did they really need the trouble of bringing up children until those children could marry? And the question is intensified because the geirim could not marry Kohanim!