One of the fun things about dating someone you've known for twelve years is that you have lots of memories of each other.  Most recent memories:

When I was in M's freshman class, I recall, early in the class, him making some remark about an old song--and how it was about giving (or getting) a blowjob.  I was 18 and impressed.  "So this is college, eh?  Sexy professors talking about blowjobs in class?  Clearly I'm on the right path in life."

I shared this memory with M, and he swears he would never talk about blowjobs in class.  Unless it was something in a novel, in which case he might talk about it, just to, you know, bring it to the students' attention.  Anyway, he denies that he talked about blowjobs when I was in his class.  Clearly, he's a liar.

The most recently revealed memory:  M told me last night that, eight years ago, he may have masturbated while thinking about me.  And it may have been to some image of me wearing a very sheer blouse.  And this might have occurred during the time period when he was supervising me.  

All possibilities.  All probably true.