Thanks for writing. I appreciate your need to share on these matters.
I would say that the main reason guys into mansex have issues with Catholic teaching on sex is that neither the appetite for nor the practice of mansex is supposed to exist according to Catholic moral teaching. No matter how you read the Catechism it still says that homosexuals did not ought to exist. Such teaching just flies in the face of scientific and empirical fact. Men exist who need sex with men, so they have the appetite for it, and that they should fulfil rather than become human wrecks.
I see monogamy as concerning the marriage of one man with one woman for the purpose of begetting and bringing up children. In the Catholic definition of marriage, the will to have children is essential to validate the marriage contract. So that the sin against marriage is adultery which has the character of infidelity to the contract made with the one woman but is equally about the begetting of children in the wrong context for bringing them up correctly. Essentially the sin is about procreating with another woman. An underlying context, of course, is that sexual activity is inseparable and indistinguishable from procreating. Sexual activity man to man is just not foreseen. My contention would be that it ought to be.
The problem is, (and you will find this somewhere on my blog), that women ought to enter into the marriage contract with the understanding that their man is going to have sexual activity with other men. But society is not there yet. Certainly, if the possibility of her husband entering into sexual activity with other men is not accepted at the time of making the marriage contract by the woman, she has the right to consider her husband as acting unfaithfully if he does so. And yet you can see that the viewpoint can be very subjective. I have heard of women who charged their husband with infidelity because he masturbated or watched porn.
For me, as you will see all over my blog. Mansex is totally distinct from procreative sex and sex between men has a totally different signification from sex between a man and a woman. The root meaning lies in the male to male bonding. This, manifestly, is more necessary to some males than others. But mansex needs discovering as a healthy value for society and even, perhaps, for married life.
For you as a married man it must be very difficult, like you say, to deny yourself your appetite for sexual interplay with men. But, if she has found out and objected through inadequate understanding I guess you are rather at an impasse for the moment. I know for a fact that there are homosexual venues where many of the clientele are married men. You can tell, in particular, when the busy time in the place is between 4.30 pm and 7 pm on weekdays. Manifestly, married men on their way home from work who cannot get to such a rendezvous later in the evening.
Personally I do not see that as infidelity to their marriage vows, and the men might not either, but their women might see things differently. I have no doubt also that many of the men, especially if they are Catholic, have a guilt complex about their activity in this sphere. I would like to see men freed of every feeling of guilt in this respect. But that would demand profound changes in the attitude of society in general. Things are moving, but very slowly, in the right direction.