My first impression of Thunder (from both the OP's description of him AND the way he comes across in his own post) is that he is emotionally low-maintenance and relatively drama-free. This is something many folks are typically not used to and don't know what to make of it when they are confronted with someone like this in their intimate lives. If this "poly-family" continues for an extended period of time to the point where you are dealing with REAL real-life issues - illness, accident, child-rearing, death, etc. - I predict that Thunder will be the rock you all lean on during the darkest of times.
As Mono would say, "I could also be full of shit".