Oh in that case, yeah, I may go out with him. I think it would be a little bittersweet (is that the word).
It's kind of like ending your high school or college relationship. Move on with life and then ten or twenty years later meet again and start a relationship. Oops really it's like knowing that guy in college but never dating him because he was taken and then he becomes single two years later. I would do it if I can really see him as my partner for life. If it's just some other semi-serious relationship then I probably would not bother. It really depends on my stage in life at the moment.
It also depends on how he treated me in the past. Did he really have an opportunity to date me and dismissed me for her or did I just appear unavailable at the time?