“There will be more butts on bikes — and that’s a good thing.” That’s probably the most memorable line from the speaker’s podium during last week’s Ford GoBike launch in downtown Oakland, marking the first-ever arrival of bike sharing in the East Bay. But some supporters of the system say they’re thinking not just about how many butts, but whose butts, will be on the 1,500 or so shared bicycles expected to be rolling in Oakland, Berkeley and Emeryville by Labor Day.