I apologize if this has already been mentioned, but have you ever sat down and told him what you are saying here? Have you said that you don't like the way he treats the girls and given him examples of how he has said hurtful things to them when with them and how he has ignored them? I honestly don't think that if he really loves them, as you say he does, that he would want to leave them so easily.
Your girls are a bit older - have you asked them how they feel about some of this? Does your 10 year old tell you that it hurts her feelings when he acts or says something hurtful to her? Have you asked them if they want him to spend more time with them when you aren't home? When I was that age, I wanted my alone time - I loved my Dad to pieces but I didn't exactly want to play board games with him when my Mom wasn't home. If he is meeting their needs while you are away and is telling them that he loves them (and he means it, especially in their eyes) then I don't really see the issue here. yes, *you* would like him to do more quality stuff with the kids - but marriage is a two way street, and with kids your age you have to take into consideration how they feel about things too. If your 10 year old is telling you that it does bother her when he says that stuff, or if the girls genuinely want to spend time doing things with him when you aren't home (and since you have girls that age I'm assuming you have a pretty good B.S. meter to see if they are lying and just saying that for you) then I think you have some issues that you need to talk about with him. They might not be issues at all to the girls though - and if you are really doing this 'for them' then you need to make sure that this is what they would want too...
My feeling (and I'm sorry for putting it this way, I don't mean any disrespect) is that you are having a bit of a mid-life crisis and you are sort of bored with your marriage. You love your husband but you aren't 'in love' with him anymore. Other guys hit on you and you start to think about what it might be like to not be tied down... and honestly, I think that most women (and men) fantasize about that after 12 years with the same person. I don't think that it's uncommon at all - it's just something that you need to reconcile within yourself. If you truly love your husband and feel that he loves you and provides for you and your family, then I think you can make it work. It will be tough and you'll have to stand your ground - but it doesn't sound like you have really tried yet. If it doesn't work, then hey - you gave it a shot, and now you know that you won't be happy with him and that you need to get out of the marriage.