I agree with this too. Boundaries.
Just wanted to share: when I was younger, I was doing a summer internship in my field and was incredibly attracted to my direct supervisor. It seemed that he returned the feelings, and many a fun time was had as we lightly flirted and talked. But I was in a relationship and never considered anything beyond flirting, even though he was incredibly attractive. Plus, he was ten years older than me, and while that wouldn't be a huge deterrent now, I was 19 at the time, so I was very technically a teenager.
About a week before my internship was over he asked me to dinner -- which, as far as I could tell, was a date. I turned him down the best way I knew how ("Dinner? I have dinner plans with my boyfriend, would you like to come along?" Not the best way in hindsight, but I was young.) Surprisingly, I was completely skeeved that he actually acted on it and couldn't look at him for the final week of my placement. The horrible thing was was that I wanted to return for another summer, but knew he was going to be there too, and couldn't bring myself to do so.... I still don't know why it was okay to crush on him in my mind, but not OK to actually date him. I think it was because of our difference in status, and of course my relationship, which he fully knew about -- even more questionable.
It turned out OK -- I got another internship the next summer and managed to turn that into gainful employment after my undergrad, but I wish that I had acted differently at the first internship, so it could have remained a possibility. Never forget that even though he may be a former professor, you could still have to cut ties/ give up a professional opportunity if one of you isn't completely mature about it. In my case, it was me. Oh well. Lesson learned. For me, it was: have crushes -- you can't help them anyway -- but there's a huge difference between reality and fantasy!