Well, I'm a professor in my late 30's...so I'll tell you what goes on inside his head. It might be just sex going through his mind, but I don't think so.
What is likely going through his mind is that you remind him of his youth. When he was in college, he might have dated someone like you; someone who he didn't pursue to the point of commitment because he was doing his academic thing and also having some fun. He probably developed a serious crush on the girl he knew back then. Now, he's all of a sudden 40 years old and life went by or so it seems. He asks himself: "Can I recapture the moment or am I too old?" He figures that he is still young enough that he might have a chance.
You seemed surprised that he "gave" you an "A" which tells me that you didn't think that you earned the A...which means that he is laying (no pun intended) the groundwork for something. He gave you an "A" and you gave him your email address.
You are both playing with fire, but what the heck...you don't lose. The worst thing that happens to you is that you get your heart broken. The worst thing that happens to him is that he ****s up his career. Apparently, he is willing to take the risk.
I can only tell you this: when he isn't your professor then he becomes just another 40 year old guy. Don't look for inspiration. Don't look for strength (because the strength you did see was him in front of the class). Don't be disappointed because he's not the same guy that you fantasized about from the front row while, probably tempting him with a low cut blouse or nice legs...and he didn't look, but you knew that he wanted to...and that made it all the more fun, didn't it??
Have a good time. Just don't **** up his career. Be nice, okay?