Flirting: Definately wrong in the context where either of the participants is married, or in my opinion even in a relationship at all!
I will say though, as a few of my good friends having been guys and though I also became friends with the women they married, have a close friendship with the men. But I don't cross any boundaries with them or do anything that I think would interefere with their marriage. I always make it clear with my words and actions and prayers that I love the both of them as a couple and want their marital bonds to be strengthened daily and with God, certainly not weakened.
Though this doesn't mean that I can't still have long discussions with my male friends who are married about various things and have close friendships with them. If we got along in spirit and in truth before they were married and while they were dating, Just because they are married doesnt mean they can't still be my close friends. Of course there are a couple extra boundaries that may not have existed fully before but other than that I don't think it's a big deal. I think too many married couples ditch all their single friends after they get married and become one of those "married couples" I am thankful for my married friends that Don't do that, atleast not yet

And from what I've seen, their marriages are healthier because of it. Because they are learning to balnace relationships with others (yes, even of the opposite sex) with their relationship which OF COURSE come first. But its not healthy to just shut yourself off from the outside world. And I think it's selfish to only choose to hang out with other married couples after you ar married. Because the single people need to glean off of you as well and learn along with you and, gasp! may even have some good advice for you at times.
That's my .o2 cents
God bless