The mat and sparring leads to an expression of your inner self, often without the conscious involvement of your intellect - you sidestep all the lies you tell yourself and little games you play and you are emotionally naked on the mat. In a sense, the parts of you that are most deeply held will come out, and you might find a martial arts teacher and a life teacher telling you exactly the same things (you don't engage, you're too aggressive, your lack of flow reflects fear of being hurt, etc.). IMHO, what you're experiencing is a very intimate interchange with sparring partners in whatever art your training in. With some of the arts that are particularly aggressive, I would imagine the sexual charge can become quite potent. As for watching men spar - that's another story. Although not part of that intimate connection as a voyeur, watching two men struggle violently can be incredibly sexual both from a physical sense, but also from a place of dominance... hoooootttt.