AN ACR FLC FLYING B737-200 AT BWI TRACKS INBOUND TO THE ARPT ON THE WRONG PARALLEL ILS.

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LAST DAY OF A 3 DAY TRIP. FINAL LEG HOME TO BWI. WE WERE CLRED FOR AN ILS APCH TO RWY 15R BUT INTERCEPTED AND TRACKED THE ILS FOR RWY 15L. I NOTICED THE ERROR ABOUT 5 MI OUT (WE HAD JUST ACQUIRED THE ARPT VISUALLY). SHORTLY THEREAFTER; TWR CLRED US TO LAND ON RWY 15R. I DIRECTED THE FO; WHO WAS FLYING; TO LAND ON THAT RWY; WHICH HE DID. I SHOULD HAVE NOTICED THAT THE FO WAS USING THE APCH PLATE AND FREQ FOR RWY 15L DURING THE INITIAL APCH BRIEFING. WHILE HE GAVE HIS BRIEFING; I WAS DISTR BY A FREQ CHANGE TO A VERY BUSY ATC APCH CTL SECTOR. I SHOULD HAVE REQUESTED THE FO TO REBRIEF; BUT I DIDN'T. THE SECOND CHANCE I HAD WAS WHEN I CHANGED FROM THE BWI VOR FREQ (SET FOR DME DISTANCE TO THE ARPT) TO THE ILS FREQ. I WAS LATE DOING THIS; WAITING UNTIL WE WERE TURNING ON FINAL; AND INSTEAD OF USING MY OWN APCH PLATE; I GLANCED OVER AT THE FO'S NAV FREQ AND SET IT. BAD DECISION. IT IS ALWAYS BEST TO USE A SEPARATE SOURCE OF INFO WHENEVER POSSIBLE. I HAD THE CORRECT APCH PLATE OUT AND IF I HAD USED THAT SOURCE WE WOULD HAVE CAUGHT OUR ERROR MUCH SOONER.