WHAT BECAME OF THE GIRLS?

The very last scene shows the girls waving goodbye to the boys who are back on their ship.
And then?
Well, I like to think that they drove to some diner in Hildy's cab for coffee and to chat about their experiences.
I can also imagine their writing to the boys and maybe keeping in touch with each other for a while.
But, eventually, they would have gone their separate ways, as they had nothing in common except their dates.
I imagine they may have met and married other men.

As to the boys' future, the movie IT'S ALWAYS FAIR WEATHER, seems to give a good description of how their feelings for each other may have changed (although, except for Gene Kelly, the cast was entirely different).