Since you say extramarital affair, I will assume you are married and the girl is not, and younger than you. 
My thinking is that if I am married to a filet mignon, why do I stop on the street to eat hamburger if a better meal waits for my arrival ?.  Now, the proper answer will arrive to you when you have your wife (or intended one) properly classified.
Since I married a former model, 175 cms tall, 52 kilos, and a trule prime beef, it will be very difficult for any other girl to succesfully tempt me, I know what I will loose if the affair is found out.

I'm married so, no. BUT were I single I'd still say no. You never know what kind of crazy drama could happen in this country. The woman could tell her husband you raped her if he finds out and to prove it could proceed with charges, the hubby could have friends in the local police force and get you deported on trumped up charges. Just find someone else, man. 

I would not go near a married Chinese women...nevermind summed up the reasons quite well.
Single Chinese girls on the other hand...if I was married, which I am not....might be tempting though.  But if I had a wife like HappyExPat's I probably would not!

If you are married, Johnsz I hope ur man enough to be loyal to ur wife. I mean, come on you are married! Think about how this stuff would affect ur family life.

If she's married- stop. The are loads of single ladies in China. U don't wanna get deported just because u decided to be an adulterer. 

Long and short of it: Think straight.

Not up my alley. Married or not. Play with fire you can't cry about it later if you get burnt.

My personal belief is one-on-one. It's always been my way, never once thought about EMA'S  or cheating because simply thats just what it is. Male or female would you like your partner or spouse cheat on you? How would you react!

I have done this before. I can tell you it is not worth the bother. In my defence I did not know the woman was married until the damage was done. If I had known from the start I would not have done it. There are plenty of good looking SINGLE women here. Take the advice of someone who has been there the arguements, phone calls , death threats are not worth it.

No way for me, either way she's married/I'm married (which I am). Family for me is an inviolable concept.

Your asking for an opinion. I thought about that once. Overall it's a bad move. Emotional evolvement is the best reason. Nevermind brought out another good point. If your thinking about an affair then your thinking SEX. So your likely to have an affair even if told not to have one.  Go get a high priced hooker. No emotion, just business!

Shit bro! just nail her. If you don't I will!

Both no's for me:

If I'm married, I married for a reason and I have integrity dammit. If there are problems in the relationship then communicate, it's amazing how many things simply honestly talking to each other solves.

If she's married, still no, too much potential trouble down the line.

NO! No one needs that much crazy shit in their life.

Definitely no. One of you is married, you do not say which one. If it is you, you have already made a commitment to that person. Are you now saying that you are not man enough to honor that commitment and you are so weak willed that even a dick can control you. If you are single and she is married why even risk the bother, you have no idea what her husband is capable of or the the connections he may have. with so many many beautiful single girls around why invite trouble.

I've done it twice in China. Do I regret it? Yeah, a lot.

Don't do it. It isn't worth it. No pussy is worth your life.

I'm not married but I never did and never will cheat on my partner, she is exceptional and I am 100% sure that she never cheated on me, I love her way too much to ever think about it.

With my exs (even in China), that would have been different.