The difference between real physical life and one's thoughts is of course almost immeasurable.
Coitus in any form is nothing more than shaking hands. It is two pieces of flesh being rammed together .. It is the emotions, the morals, the beliefs, the personal needs and so forth of the individual, which differ. It is those inputs which make sex something personal to the individual.
If as it appears you both intend to stay together, then you must both tread with the most extreme care. Before you start with all the multiple partners etc etc ... You and your husband really need to understand where he is at, where you are at, plus where you stand as a partnership in life. Then, what about the effects on your children? Because unless you want to bring up two anxiety filled psychotics, it requires thought.
After my own 40 years or so owning adult sexual entertainment businesses, there is little I have not seen .... Your story is a long way from unique.
However, the internet seems to myself to have opened a whole vast can of worms for many. People work themselves up by watching the endless porn ..... But would your husband have searched it out pre-internet? Would you two have gone to a swingers club and so forth? Maybe, but maybe not. People take these life changing steps, IMO, because it is there in front of them.
Myself I am around 60yr old, my wife early 50's ... we have 30 years together ongoing but are in effect much the same personalities as we were 30 years ago, happy and solid together ..... Your husband has to make adult choices regarding your children and your marriage. As do you.
Sex is unemotional but the effects it has on the 99.99% is entirely emotional ... If he has a male lover it is no different to a female lover ... He could become fixated with the other person. The other person could become fixated with him. It is inviting real danger of collapse into your marriage.
Therefore my advice is realise that you both are standing in an unmapped mine field .... That you both need to be extremely careful
where you step. Standing still is of course a positive action.