Ok...so what do you do to help redirect that when you thinks she starts hearing nothing but that?
Have you ever asked her to in her own words repeat back to you what she thinks you are communicating (this can be done in a non-condescending manner). Have you looked at her and said "Honey, this isn't about sex. It's about how I feel about you and how we relate with each other."? Different things like that. Because when you get right down to the brass tacks...it isn't just about sex. In this case there's much more going on....if all she's doing is just showing up, or opting out when the time comes. You have other issues going on too...perhaps she doesn't feel close to you (for some reason), perhaps she has multitudes of things running through her mind and she doesn't put you and your M as a priority....there are many things that could be going on.
Ask her....flat-out why she made an agreement with you to have "appointments" if she thinks she can opt-out. You don't tend to just treat "appointments" in other aspects of your life that cavalier (or does she?).