hm. Reminds me of a situation I had with a teacher who sounds awfully familiar. He helped me out, was just a real nice and funny person, and sometimes we got to hang out. This sounds kind of perverted to begin with, but we're like a big family at this part of the college. Heck, another lecturer was our DJ at the halloween party. It's a great atmosphere, because the teachers get to be humans, not 'bad guys' who make us do work. They're such a funny bunch.
ANYWAYS! To conclude though, I never had any feelings for him, and even if I was single and I did have feelings, that's still a bad road to go down. I don't recommend it. The age gap nor the student/teacher complication don't matter as much as the fact that he's cheating on his girlfriend, and even if you say you don't care about that, if you get yourself into a little fling with this geezer it'll be harder to move on to a proper relationship. And he could loose his job.