It's probably because she has some slight insecurities and doesn't realize that even though you were being honest, you did indeed see her "hot" enough or whatever to date. . haha however it is harder to understand why she got upset considering she is an ex and not your girlfriend haha :)
They call that a double standard. Women are allowed, men are not.
Lol, my hubby and I talk about hot women. We both have a crush on scarlet johansen She's smokin hot.