I know plenty of people who say sex with someone other than your wife, or with a married person, is adultery and a Sin against God and a sin (lowercase) against your wife or the other person's spouse.

I know some people who say thinking about sex with a particular individual other than your wife, or thinking about sex with a particular person who is married, is adultery of the heart and a Sin. Jimmy Carter admitted to this Sin.

I know some people who say that obsessing about sex is a sin, just as any other obsession (other than an obsession for following God) is a sin.

However, occasionally thinking about having sex with a generic person other than your wife, that I haven't heard many people say is a Sin.

Here's my take:

If an act, any act, separates you from God, then it is a Sin against God.

If an act, any act, separates you from someone else, then it is a sin (lowercase) against that person, or more specifically, against the relationship and/or covenants you have with that person.

In your case, I would say thinking about specific boys while you are having sex with your wife could be "adultery of the heart."

I would say thinking about generic boys is not a Sin against God.

However, because you know she doesn't like it and because you love her, I would say that thinking about boys sexually and making no effort to reduce this is a sin against her and your relationship with her.

Yes, I know this goes against your nature. Yes, I know it is hard to go against your nature. However, you aren't the first: Many men think about other women when they have sex and if they are to be true to their marriage they are obligated to do whatever is reasonable to put those thoughts out of their mind. True love is giving up part of yourself for the benefit of your partner.

Now, I have a few words for your wife:
You married Cat. You married him for better or for worse, in sickness and in health. You may not have known it then, but you now know he is a pedophile. You now know that it is an extreme hardship for him to attempt to put these thoughts out of his mind. You know that by asking him to do so you are causing him pain, a pain that will never end. Please rethink your stance. If you love him, then please, out of love for him, find an acceptable middle ground. Split the difference. Be willing to accept some pain for yourself knowing he has these thoughts in exchange for reducing some of the pain you are asking him to endure.

To both of you, part 2:

Seek professional counseling, preferably with a marriage counselor experienced in counseling couples where one person is a pedophile.

To both of you, part 1:

Stay in constant prayer over this. Do this for the rest of your lives. If nothing else, praying will bring you both closer to God and to each other.