Perhaps the SJs could say a little more to this with regard to their personalities, but as an ENFP, I can say I think libido is highly unrelated to how conservative one would actually be in terms of partners or openness about sex. In fact I've seen a lot of people who I thought had major poles up their asses just to find out that when they talked to me about their sex lives, they were the least conservative of all. In fact many of the people who talked and talked about it tended to be less like that. *shrug* I know this because people talk to me all the time about this kind of thing.
Oh well, maybe I should clarify. Although it is highly unlikely that a person with limited libido would decide to cheat, I do not think having a moderate to high one would make a person cheat. But maybe the SJs can say if that is accurate for them.
Perhaps SJs have even more libido than any of us but they can keep it under strict control.