Heck, if they didn't, then you'd also have to believe Clark was cheating on Zara by continuing his engagement to Lois after learning about New Krypton. Yet I've never seen FoLCs say that he should have broken up with Lois once Zara came to town.
I think this is more because we see no reason at all why Clark should see as binding on him a relationship he never willingly consented to.
On the other hand, I am not sure anyone has seconded Clark's view that he should hold off from relationships because he is still legally married to Lana.
I would however point out that it has been less than a week since Lana told him that she was in a relationship with another man and leaving him. He has not really had time to emotionally move beyond the married outlook. So I don't think it is reasonable to expect him to be willing to jump into a relationship with a woman at this point. Also, since as far as he knows she is married, I think the "we're both married" is more to try to shock her into acting properly than anything else. He is very uncomfortable about being the man leading a women into adultry, being like whoever the man is who Lana went to, so he is doubly reluctant in this case.
He may have overreacted, but I would find it hard to see as in-character acting if he didn't.
