I remember having 2 girls on the go when I was about 20... It was pretty funny. They both sounded similar over the phone, so when they rang I had to be careful...
At one stage, one of the girls asked me, "are we going out, or just bonking?". Tough questions! haha.
Jeepers, it was a balancing act at weekends! At least I had surfing and windsurfing to get away from it all.
The blonde one eventually found out, on the grape vine I guess... She came over after finding out, to personally tell me I was dumped! But she was also still happy to have a good buy spanking! Sad thing was that she was probably the keeper out of the two.
But at the end of the day, it's not fair for anyone! Funny days, no serious harm done at that young age!! 
Maybe remind him that his wife will find out sooner or later... And then his world as he knows it will come crashing down, and he'll be paying her out with a settlement, she'll keep the house, he'll need to rent a place, or move in with the other girl... It's gunna get messy !!
. 
You'd think that a principle is a principle...
This thread reminds me of the girl saying: "I would never engage in prostitution, it's disgusting. Well, for $1000 maybe".
Either you're against cheating, by whomever, and then you take action.
Or you keep pretending that it's wrong and in fact you condone it.
Well hoping to inform on today's events - but everyone bailed, 1 for work and othe had a problem with there kid.
Now I am really struggling - my head is doing back flips. My now preferred choice is to ditch him ( due to alot of feedback and I know Karma is going to bite me really hard).
After a brief conversation to him today, I now am well aware I am being used.
Like my Nana said when I was 18 - everyone thinks they have friends but wait until you need help, not many will ever come through (he always has). You should be able to count them on 1 hand any more and there just associates. Hence why I only have 2 friends and a few associates.
I am personally lost right now, in a bad place...
. Thank heavens I have my girl to talk to as all my family has passed away and both these guys have helped me through that..
I find trust a big issue in friendship
I wouldn't call someone who betrayed another person a mate.
Loose the best friends. Your friends cant be that close if you don't know his misses.
You have to distance yourself with this idiot before his problem is apart of yours.
People change........maybe he has helped you regarding the loss/grief/anger stages associated with death of people you care about but that was 'positive social support' he offered as opposed to the 'negative social support' that he is seeking from you.
The other thing to consider is does the other woman involved know that he is married? If she doesn't, then that means he is 'using' , or putting his needs (whatever they may be, he probably needs to work that out himself) ahead of 4 people and lying to himself..........
Perhaps you could explain to him, in very blunt terms, how his relying on you for an alibi is making you feel like a weasel, then suggest that he is welcome to get in touch when he has sorted his baggage out.
A similiar instance happened with one of my wife's" best "girlfriends who used going out with her as an alibi to her having an affair, my wife to her credit upheld her own moral values, said stop, I want no part in this charade to her "great" friend and it cost her the friendship. It hurt her and probably still does, all it did was prove how shallow her "good' friend really was... and could be true about your good mate....
I've gotten kinda disillusioned last few years. Bad relationship experiences and know so many friends - male and female - who routinely cheat on their husbands/wives and don't think twice. I have felt like a naive fool and kinda stopped believing in love and fidelity and all that. While not a thread I'd usually read on SB, it's really refreshing to see so many people stand for values I thought were maybe dead. Thank you very much - has kind of restored my faith in love a little bit. Nice to think maybe those values aren't too much to hope for
Good luck YB. Takes lots of courage to stand for your values - especially when it's a mate who's compromising them 
Don't confuse supporting a mate with what this guy is doing...
He is cheating on your friendship almost as much as he is cheating on his wife.
I have a guy who works for me who has just found out that his wife is cheating, he is absolutely gutted, his ability to work is a day to day proposition. I almost feel I am on suicide watch half to the time. There are three young kids involved in his case. Seeing the affect on this guy I personally could not be part of anything that will eventually affect another person like this.
Like Hamsta said, let him go. Often friends are in troubled places and we need let them go through it on their own and simply be there for them when it all falls apart. You being a part of his deception is causing you so much pain that at a certain point you just have to cut and run, protect you and your own. If he's a true friend he'll come back to you once he's sorted himself out.
Also, he's been cheating for a year! Either his wife is incredibly stupid or she already knows. There's not much a wife will miss - even with kids keeping her busy. If I were you I would make it clear he can't use you as an excuse any more - if she knows then she will know you're keeping it from her as well.
Good luck! I don't envy your position.
Cheers guys and gals for some of the positive reply's they certainly are helping to understand his irrationality.
I have tried all week to catch up sit down and talk but he is apparently to busy.
After talking to another friend and him it sounds like she is definitely onto it, she has started to notice the changes in him and gave him a surprise visit and caught the 2 talking. So tempting to send her a message "your getting warmer" but but... yea I'm not sure.
I am now only giving him a week to confess or his lost a great mate. After a brief talk this week I did explain you can not use me as an excuse anymore as I also associate with one of her girlfriend and husband, he understood that as our hobbies/interest always aligned
The more I read and think about it the sicker I feel for not telling her and knowing what he is doing - almost ashamed - but not far off
The girl he is dating does know he is married and does not care (bitcx) although I have not seen her for about 3 months she is always trying to get on my good books through facebook - I dont have a bar of it and deleted her of my account.
I hope he does get caught out now( they don't own a house) and she pillages all other possessions. I am staying reserved for 1 reason now and that's their child - I hate broken families as I come from one.
To be cont.......
Final update.
She broke up with him as he just was a prick to her for the last month.
He has not told her he is/was cheating, just a cop out way to get divorced. I think he is scum and ditching him as a mate.
Only bummer is my decision to ditch my other mate aswell as he supports his madness and they are shacking up together. That decision was made by everytime he gets a new girl you don't see him for 3mths - the second she is gone it's full on lets do this - lets do that, then BAM gone again. Also the cheating ex mate BS's me about I need to do this and that, to busy to catch up - cause he is to busy shagging this new hoe.
Did not think 2 friends for so long could do that to her/me. So sick of being used, so they can all get Fxxkxd.....
Cheers all, you guys where more on the money than me...
A good result youngbull.
Let us all know when she is ready to get back into the game 
Young Bull, it sounds like you have followed your values on this one. Not everyone has the courage to do that.
Best direct your energies to friends/family that share your values.
Good luck
Sailhacks right.... in time they might come around..... people can learn from being pork chops... and even become better people from it. IMO.
Sounds like she puts out, start tapping her as well, that way you guys can hang out some more
Let us know how the divorce goes.
You were saying she can be a tough biatch?
It sounds like one of those cases where the guy deserves a bit of it... (from what you're saying).
