Is it a mortal sin for for a spouse to love another person without dating them?

No.

So long as you NEVER act on this "love" there is no mortal sin. No phone calls, no conversations, and certainly no touching of ANY kind -- not even a "friendly" hug.

In fact, WHEN you make a conscious effort to ignore this other love -- because you know that it IS immoral -- You will find that the hidden love diminishes and disappears.

Yes. Being emotionally intimate with someone can be as devastating as a physical affair. If your affections are elsewhere, it is difficult to give your full attention and affection to your spouse and your marriage will suffer for it. Where the mind goes, there goes the body. It might not be physical now, but it is definitely playing with fire and a disaster waiting to happen.

P.S. -- I presume you are not talking about love like "love your fellow man" or "loving your mother." You mean love as in emotional, intimate love like you SHOULD have for a spouse, only for another person.

No its not. Loving another person without dating them is not the same thing as forsaking your spouse.

The idea is you forsake others to be with your spouse. It doesn't mean you have to shut your emotions down completely or stop loving someone else. If we were to be this indifferent to everyone in our lives whom we love, then the world would be an even colder and more desolated emotional place.

No, but 'adulterate' means to dilute something, and while you may not technically weaken your engagement with your spouse while thinking of someone else, you can't help but dilute your emotional bond. Strengthen your commitment to your spouse, and love others as friends, not as partners.

You should love the person you are married to. Thoughts of love for another is the devil planting thoughts of lust in you mind. You should talk to your minister about the problems you are having so you can pray for help to heal your marriage.