When are sexual thoughts sinful in marriage?

I read recently in a book on scrupulousity that thoughts of a sexual nature are not always wrong in marriage because it’s part of God’s design for marital union, but the author didn’t elaborate much on that point, and I can’t seem to find any guidance on this. The assumption of most seems to be that thoughts like this are automatically considered impure, but I doubt that, and I’d like to have a clearer understanding of where the line should be drawn. When is it sinful and when is it not? When it’s about your spouse versus someone else, or when you’re in the moment of the act versus at other times, etc.? How do I know if I’m lusting after my spouse or not? I am very confused.

There is no exact line in the sand, its more a matter of emphasis. Sexuality is not sinful, it is part of marital love.

The most pertinent questions to ask are:

Is someone’s sexual thoughts the result of an expression of love for one’s spouse? Or do the thoughts merely reduce one’s spouse to an object of pleasure?

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