I know it's not your intention, but what happens often is that single male gets 16B and his girlfriend gets 24E. If they're foreigners, you probably wouldn't even be aware they were separated. If their names are different (ie they're not yet married) the ground staff and cabin crew wouldn't even be aware they were separated and, as a check-in agent, I have no power to juggle other pax seats around to accommodate them. I'm glad you can now see the dilemna - I know you, and 99% of the pax I have hitherto branded as "selfish" are not being inconsiderate deliberately. Unfortunately, 3 row seating is an unhappy arrangement - perhaps someone could tell Boeing and Airbus!
I understand the problem, but I'm not so sure about the shortage of solutions.
On every occasion when a deliberate use of the window/aisle "trick" hasn't worked, I've chatted to the singleton pax who's turned up - all genuine singletons. Of course, if the trick works and the middle seat stays empty, it follows that no couple has been split up. Check-in ought to be able to identify most couples in advance, because most will be travelling on the same booking.
I've once been part of a group of three who had the benefit of a row of three all to ourselves. Until we were split up at the gate because a Gold Exec Club member wanted to canoodle with his wife. It was only EDI-LHR, and we had been so late that it was a miracle that we were accepted for the flight at all, that I thought that the argument wasn't worth it. But it was intensely irritating to have three good boarding passes (issued about 90 seconds earlier) taken away at the gate by an agent who had taken it on himself to do all this rearrangement manually.
And we've all seen cabin crew negotiating with other pax on board to swap seats to get couples together.
But I agree that if there is a growing problem we ought to be sensitive to it.
Obviously, sometimes airlines have no choice about how many seats in a row. But I'd understood that airlines picked 3-in-a-row seating where they can when they expect a higher proportion of singleton pax, to maximise the number of empty middle seats. 2-in-a-row where possible is better on routes (eg leisure) where you expect a higher proportion of couples.
Hence BA's 3-3-3 777s, compared to AA's 2-5-2. On the BA aircraft, if all pax are singletons, then nobody needs to sit next to anyone else until load exceeds 66%. On AA, if all pax are couples, nobody needs to sit in the dreaded middle seat until load exceeds 88%. And doesn't someone have a 3-4-2 layout in a 777, to minimise problems?