It is out of respect for their relationship likely.

Their spouse may want it this way.

I know when my relationship with my boyfriend got more serious I treated my own internet friendships differently. Some I even ended entirely. Frankly, I did so out of respect. Remember, it is your partner who should be your confidant not someone on the internet. Please realise this can be incredibly threatening for some people. Frankly, I am one of them. I really don't believe someone who is in a relationship should be carrying on with someone else - and that is what I believe this friend and you have been doing - regardless of how emotionally intimate the relationship is. I don't feel it is right. Obviously I have pretty strong feelings about this, yes I do. As much as I was prepared to cull my own acquaintances, I should say I also expected him to do the same. I am his confidant not someone else. And I feel even stronger about this if such an acquaintance were a woman.

I feel strongly about this.

This is about his respect for his future spouse not about you. Sorry you are caught in the crossfire.