I’m really glad you posted this because I have the exact same problem myself and I would like to find out the answer to this question.
The problem is that the responses are either off topic or not credible. I’m not sure you can ever find credible responses to questions in this kind of forum.
I’ve had priests say they were okay with this, I’ve had priests ignore it, I’ve read things online that were from books written by priests that said it was okay. But, you and I both know that some of these thoughts are certainly outside the realm of what “nice” girls,. or maybe anyone, should ever be thinking.
The problem is that thoughts that are not “dirty” or potentially sinful, are also not arousing.
Thinking of yourself and your spouse wrapped in a beach towel sipping a cool drink is hardly the prelude to red hot sex, nor is giving yourself to him with full realization of his magnificent dignity as a child of God. Get real.
After decades of marital fidelity and physical decay, it is very difficult to get aroused by the same old same old. In order to enjoy the act you almost have to use your imagination. Maybe you aren’t imagining a “real” other person, maybe you are just imagining a person or persons who do not exist. Maybe you are imagining a fictional couple who is physically attractive that isn’t you or your husband. Is that a sin? I mean, I could go on and on with hypotheticals but please, let’s not state the obvious. I’m sure this is a gray area. Please don’t continue to respond with some nun-like answer. If you don’t know for sure, just don’t respond to the question. The answers so far show a great lack of insight.