errr, you have locked him out and made this a crisis?
Instead of making this a crisis, you need to just take some time to think.
I suspect that lots of men and women do flirt with people of the opposite sex, all the time and some of the time, and not in front of their partners, and sometimes with their partners knowledge. Sometimes friendly banter and flirting is virtually indistinguishable, sometimes what is just friendly banter steps over a subjective boundary into flirting. When that flirting is unwanted, it becomes something else entirely, again often this is subjective according to the receivers personal experiences and subjective thinking. But it may in the final analysis just be flirting, or friendly banter.
The world in which we live; men flirt at their peril (IRL), because we perceive even mild flirtation from an unworthy source to be outright harassment, and mildly flirtatious behaviour from married people to be evidence of betrayal.
What the internet seems to provide for these silly men, is a seemingly safe place in which the banter can become flirting without the threat of people calling "harassment or betrayal" Of course it may be that your husband is one of these men? and seeks all the while to push those boundaries and this is the real reason he prefers to befriend women and not men. He can't flirt with men and besides he wants to be friends with women he imagines he could fuck. In fact, I would suggest that the internet lends itself to this. In the past when things were different people flirted more in RL and it usually amounted to nothing more than passing the time.
Confused? good, just as I am confused as to why you think that a few internet exchanges of a mildly flirtatious kind warrant a "crisis"
Talk to him and then calmly decide what your terms are.