Regardless of what my personal views are, I don't think it matters what someone posts on FB. People do things that I don't personally or morally agree with all the time. Even people that I love and respect. For me to say "you can't talk about it" would make me a hypocrite, as I'm sure there must be something (likely many things) that I have done that don't mesh up with others' personal beliefs. I can say "I don't want to hear it". Or I can choose to end the friendship. Social consequences are mine to dole out, but moral ones, not so much.
So, for marriage in general. I don't really care under what institute someone is married. If they say they are married, I respect the marriage. I.E. treat them as a social unit, congratulate them on 'anniversaries', understand that one may not feel comfortable making decisions or plans without input from the other, etc. I do not feel that I have a right to institute my own beliefs about how they behave, what they do, or how they break-up. Of course, a break-up of friends, coworkers, family members, etc can have an effect on me, but that still doesn't give me the right to say "You have to do it this way because I attended your wedding and have a say so."
What I can do (and will do for friends and family I care about) is say "look, you're my friend and I love you. I understand that you are trying to move on from a tough situation. As your friend, I want to be there for you. I'm uncomfortable with your new relationship being so 'out there' when you're not yet divorced, yet, I also recognize that I'm not you and don't know everything. I am your friend though and I care about you, the kids, your relationships, so can we talk about this please?"
There's nothing wrong with calling a friend out or at least letting them know that they may regret their actions later. For someone that I barely know or have no relationship with, I stay out. It's not my business and if the FB things bother me, I unfriend and move on.
I understand why someone would question the timing of the relationship in the OP. I think that's natural. But, I don't think that makes putting it on FB rude or "throwing it in people's faces".