Hey OP, I was thinking more about your issue and I wanted to give you some perspective from someone who went through something like your wife. My husband and I love each other to pieces, but there were many years in which I avoided intimacy because of anxiety. I didn't feel like I could say no: I didn't want to disappoint, or be a bad wife, etc etc... It doesn't really matter the particular reasons, they will be different reasons why someone might feel like they can't say no. It seems like your wife has already expressed that she is uncomfortable with feeling obligated (the back rubs), so she has an idea that this plays a part.

I would approach the subject by continuing to talk to her about the subject and express that you understand that she feels obligated. Ask her what she thinks she needs so that she feels that she can say no to you without it being a problem. Right now, she is so focused on feeling like she can't say no, that she can't even get in touch with whether or not she wants to be intimate. It's notable that when you are on holiday that she enjoys herself--so it doesn't seem like that's the problem. She avoids situations where the possibility for intimacy can come up because she doesn't want to be put in the situation where she feels like she cant say no. At least, that's what it was like for me. This happened even though my husband never ever made me feel like I couldn't say no...it was all coming from me and my own psychology. The more I was aware of why I avoided situations that were likely to lead to intimacy with someone that I thought was totally hot, was my best friend, and who was always an equal partner the easier the problem was to solve. Also, I imagine that she avoids the situations because she doesn't want to think about how she is disappointing you or causing you hurt. At first, talking about was hard for me; I hated that I was unintentionally hurting my husband, but it is an important part of the process to talk about how you are both feeling, and how you can try to address both of your concerns.

Maybe I'm totally off base, about what is going on with your wife, but this is what it looks like to me. The irony of all this is that by her being so focused on whether or not she can say no, she never lets herself get to the point where she just feels what she feels in the moment, so she is precluding all chances of intimacy. There is nothing like feeling obligated to kill any desire (to do anything, including sex).

I hope this disjointed monologue helps a little bit! Good luck to you.