Sorry but i cant get worked up about this but if a "couple" wants to book to go away with a singles group and pay for seperate rooms then share, more fool them i would say.
Have seen it once but to be honest it was a big group and the fact they didn't mix i never saw as a issue, I've seen many people travel together such as friends, cousins, workmates, mother/daughter, niece/aunt, sisters etc. Some of these have mixed with the group others haven't but the same can be said of some solo travellers (very rare).
On my recent Tuscany holiday i was asked if a group of us that were getting on well had travelled together, the answer was NO we had actually met the night before at the airport hotel through this forum (i've never travelled with anyone else but have occasionally known others from previous holidays, but not planned to book same trip).
On these holidays most people make friends and group up a few days into the trip, that doesn't mean they leave others out, its the way it is. Not really too much different to travelling with someone, its your loss if you dont mix.
On the 2nd type couple, someone who meets on holiday and clicks i see no issue whatsoever, we are grown ups and it happens anywhere, lots of singles together increased odds i would say. Lets be fair its very rare (less than one couple per holiday i would say)and in most cases moves on after the holiday rather than on the holiday. But if it is happening to somone and i see they are getting on well i'd just give them a bit of space.
All this makes it sound like these holidays are full of friends and couples, that certainly isn't the case the vast majority (95%+) are solo travellers.
Mick