Air Carrier landing SFO was issued a go around by ATC because traffic and speed assignments could not be achieved; the reporter voicing concern regarding the NCT Controller's technique and speed/visual separation requests.

Narrative

Vectors for visual approach; final turn to intercept. Controller pointed out traffic on right base to final for parallel runway; and directed to maintain visual separation. This is a constant issue at SFO since after the intercept of final; the aircraft we are told to maintain separation with will often be past abeam at 4 o'clock; making visual contact virtually impossible; but we do our best to comply. On this approach; we were subsequently directed to report traffic at our ten o'clock position; intercepting the final to our runway in front of us. We reported that traffic in sight. We were then directed to maintain visual separation with both aircraft; to maintain 180 KTS; and not overtake the aircraft to our right for the parallel runway. Since that aircraft was a regional jet (lower approach speed) and was already abeam to slightly behind us; and since the other aircraft was at our ten o'clock position on the opposite side; I informed the Controller that we could not comply with his instructions/restrictions on our visual approach. He seemed incredulous that we could not do the impossible; canceled our approach clearance; and re-sequenced us for the approach.