Yes, you get consecutive boarding numbers. But even if you check as the first group, you will still have B numbers.
And all the aisle seats and window seats will be taken by the people who boarded before you.
Who boarded in front of you:
Business class, travelers with frequent flyer status, higher priced tickets, families with little kids, elderly, people who paid for early boarding.
Since the SW planes I have been on are always 3+3 configuration, this leaves only the middle seats to choose from. Yes, Sometimes there have been some seats in the back where 2 of us could sit together, but it’s the exception, not the rule.
Granted, we have almost always flown on fairly busy days and routes ( around Presidents Day to Denver) but I Imagine many others here might be doing the same.
It’s just something you need to be aware of and be willing to accept. If you have kids, and are used to other airlines, it could be a shock.