How do they do the weight & balance?
The ops agents can control where all the bags & cargo go in the holds (forward & aft), so they have a lot of control of the CG that way. Generally, unless you're close to a limit, it matters not; with a really full airplane, the pax will be evenly distributed since there won't be many empty seats (& since they'll all be middle ones, fairly evenly distributed at that); on a mostly empty flight, it isn't all that much weight that you're worried about. On the rare case when you ARE close to a limit, the ops agent uses "the loading rule" (that's what the W&B form calls it), which requires half the pax to sit in the front half of the jet (not usually a difficult thing -- people want to be "first one off" most of the time anyway). The flight attendants would ensure that this happens, if needbe.
In general, though, when you consider the empty weight of a 737 & the amount of control you get putting bags/cargo where you want it, it would take some doing to get too much out of CG with just pax alone. If you were to get in trouble that way, it's probably a case of the "estimated" pax weights being off -- that would concern me more than where they sit.
Somewhat like the other transport catagory aircraft I've flown... you COULD get out of CG in a herk, but it generally took a lot of doing. 98% of the time, any rational load plan would be fine, and you would go over gross long before you'd go out of CG. By the time you had enough weight to move the CG very far, you pretty well HAD to have it uniformly distributed (not enough room to do otherwise), which helped the CG situation. Hauling heavy equipment, you needed to have it tied down in the right spot, but the center of mass of 20k of equipment can vary lot more in a fore-aft direction than the center of mass of 100 people!
Hope that helps!