bunny21, you have gotten some good answers, but it is not the advice you wanted, because I am sure you thought about all this already. If your boyfriend came back to you with no strings attached, you seem like you would forgive and forget. And after ten years, that would be a normal attitude. Many spouses take back a straying mate and move on. After all, everybody makes mistakes and acts stupid at times. We grow past it, or we don't, and who can say in advance what will happen?
So, you want him back, but the real question is, can you keep him if you take him back? Not at the moment, in my opinion, but that doesn't mean never. People change, but very slowly. Your boyfriend hasn't learned his lesson yet, by a long shot, if he wants to keep his options open. Well, it is time to start making him pay for his options. The first thing is for you to start dating other men. It doesn't have to be romantic dates, just go out with other guys and have fun. Until
your boyfriend swears off all other women, you should make a concerted effort to "play the field". You don't need to get romantic with any of these guys, since you want your boyfriend anyways, but you do need to socialize and let him know it. You have options too, and he is holding the weak hand in this game. You are worth being involved with, and he is a douche.
Sorry, I know you love this guy, but let's be honest. He may have lots of good qualities, but trying to string you along while he waits for a married woman to dump her husband? That is a little overboard for showing a lack of respect for you.
So, anyway, that's the deal, he wants options, and you have options. You hold the whip hand, and you should apply it liberally. I think it is time to teach this guy some respect and make sure that he comes crawling whenever he comes around. It is time for him to prove that he loves you.
Everybody involved in this situation wants their cake and to eat it too. The wife that is cheating on her husband is the really sick one in this mess. Once you start having kids, it is too late to back out of a marriage. When she does get caught by her husband it is going to destroy their family, and the repercussions will last for years, with a messy divorce, child custody fights, money problems, emotional distress and so on. Your boyfriend will then be involved in all this, and who knows if she will still want him then? She is not a stable person and her decisions are not rational. So, your boyfriend is setting himself for some really hard times to come. Now is not the time to make it any easier for him. It may be possible to love two women at once, but that doesn't mean it should be painless. He owes you an emotional debt for your loyalty (and why he would want to hang around with a woman who cheats on her husband is beyond me, but everybody has their own set of values), and you need to make sure you collect on it.
So, this is getting long, but let me summarize: Keep this guy at arm's length, date other men (ask them, don't wait for them to ask you), make your boyfriend work extra hard to please you, and as long as you can handle the drama, you can put off dumping him. If the cheating wife finally takes him away, count this as a blessing, and be glad that you have all those guys you were dating to choose from.