I think that they "truly" love one another but maybe they're scared to "Come Out" fully. They both have children now and from what I heard they share a Flat in London so who knows. The Chemistry that those two is nothing but amazing and beautiful.
I also feel that because they came from such "Humble" beginnings they have this closeness. They know what it's like to live in the equivalent of a Ghetto or Low Income Housing Project, not having the things that the other kids had and took for granted even finding out that Kimberley's Family almost ended up Homeless. Then having their setbacks, bad breaks, and both having Fathers who weren't really reliable that's why they are both so grateful for what they have now.
Also some people just have that "bond". Look at Otis Williams and the Late Melvin Franklin. They met as Teenagers in High School in Detroit, MI auditioned for Motown in 1960 and they were the Co-Captains of the Temptations Ship from 1960-1994 and that's saying a lot considering all of the "Changes" the group went through. Sadly enough Otis is the only sole surviving member and although he has a good rapport, I'm sure he misses his old bandmates most notably Melvin.