Well, he seems to do a fair number of little readings from the New Testament, and they do a bit of embarrassed sniggering about girls in some of their videos, so I'm pretty sure they're primarily excited by sex with females.
On the other hand, in this latest video, the frank enjoyment of "Thomas's" very lovely feet seems quite unmistakable to me: at about 3:55, when Charlie gets up to say goodbye, he "high-fives" Thomas--and then "foot-fives" him--which playful little slap of hand to foot ends with him sort of caressing Thomas's toes. (I attach two captures of the screen--the quality is too poor to really enjoy, but just to give you an idea of what this "foot-five" and "monkey-shake" look like.)
It develops into a kind of funny hand-and-foot-shake which Charlie refers to as a "Monkey-shake"--I guess with reference to the way a chimpanzee's hind feet resemble a human hand with an opposing thumb more than they resemble a human foot with a great toe that doesn't oppose the other four.
The frank enjoyment Charlie has of his friends' very, very handsome soles is unmistakeable--but I think you're absolutely right that he does not feel, at least consciously, any specifically sexual satisfaction in these activities.
It's also intriguing that his friends not only don't seem to mind these obvious attentions to their feet, but they seem to be humoring him, and enjoying the fact Charlie likes looking at and playing with their feet.
I'm getting quite hooked on this fellow, and quite fascinated with these films. I think I'm going to try to collect the bunch of them and find out a bit more about who these southern fellows are and where they're from, and maybe get some idea of what they think they're doing.
I wish wncmtnft would chime in and say something--he lives in North Carolina, so maybe he can recognize, from their dialect, where these fellows might be from.