I usually get involved in somebody's love affair..not intentionally,fyi..

I think they are old enough to handle situations and since their adult too they know exactly whats right and wrong and they should be the one to realize when to put an end to their love affair. I know your feeling and its somewhat unfair on your part to be in that kind of situation but because they are your friends you have no choice but to watch and listen to their actions. My advice to you is don't be tired of listening to their problems but you have to let them realize their fault and if possible try to let them see the outcome of their relationship. Don't consent them nor agree with them but be still a good friend to them.

I personally feel that eventhough how much you talk them out of this illicit relationship or love affair; if both of them are hard-headed, you couldn't make them stop at what they're doing. Not that you shouldn't at least try to correct them, but you could give it a push. In fact, I think you really should do your part as a concerned friend. After all, you know how each one feels for the other. Like you said, this does not involve love. And ideally, every boy-girl relationship should be primarily based on love for one another. It is unfair not only for the guy's family (specially the kids) but it also is for your female friend. Old folks would often say and comment that a father would always come back for his children and in doing so, it isn't impossible for him to fall in love all over again with his children's mom. Even if he doesn't, your female friend couldn't take away the right of the guy to visit his kids; meaning his time will always be divided between his children and your female friend. Assuming your guy friend and female friend goes on with their affair and she gets pregnant or they eventually have kids, would she be sure that the guy would never leave her? Or would he leave his first family for her? It's so sad to think that sometimes, we get into a web of deception either brought by people around us or even by ourselves and we feel we're so trapped in it eventhough the real picture is, we only got one foot on the web. Do try to convince your female friend that the world will still evolve without the guy in his life now, and that she has better great opportunities to meet the man she deserves to love and will spend a hundred percent of his time and life with her. After all, she's still single and very much available. It's worth a try.

I think sometimes you have to make your intentions known, I would sit them down and seriously talk to them and tell them you value their friendships but you are not going to get involved and you are definitely not going to take sides and that is the way it has to be...