If beach is important, than I say go to CN. The AN beach has the best beach area plus the ocean is wonderful for floating in. In the 3 times we have been to CN we have never seen any children wandering by the AN beach. I'm not saying it doesn't happen, we've just never seen it. The ocean is very clean with no seaweed and is very calm. The beach is also large enough that you can be just the two of you and not need to socialize if you don't want to. The ocean area is also large enough that it is never crowded.
If the beach is not your main requirement, then I'd go to CSS. While it has a nice large beach with lots of private areas, the ocean is very rocky and the water is cool with a strong current that draws you toward the rocks. The AN beach is large enough that you can be secluded and the ocean is never very crowded due to the cool water and rocky ground with lots of seaweed.
Of the 3 areas, I felt like CTI was the least private for couples who want to remain by themselves. The island is very small and can be really busy on some days. Plus it seems to be a bit rowdier than CN or CSS. There also is no beach to lay out on and soak up some rays. The only way to cool off is in the pool (very social) or on a small patch of beach where the boats drop off and pick up people.
As for best time of year to go, the down season would be the least crowded, but Couples are boutique resorts that are never crowded no matter what time of year you go.
You also will be pleasantly surprised at how respectful ANers are at Couples. We've hung out with some pretty amazing bodies and the girls were never felt oogled over.
Juliann & Jeff
Jamaica Soon Come