You're right. Your kids need their dad as well as a dad.
If your ex won't 'parent' ... they need a parent, and it's really hard for you to do all of it. You might be great and quite able.....but my opinion is.....just keep gently inviting your ex around.....don't 'fight' it even if he isn't following the rules, because you're likely to get nothing.
SG, you've helped me out so many times and been a mentor/great friend to many of us here, but I don't know if I agree with that. Do kids really need 2 parents fully in their lives to be healthy? I have tried everything to keep my M intact and would love to have 2 parents for the kids, but the truth is even when he was living with us he was an absent parent. My S14 said he didn't notice any difference when H moved out b/c he wasn't around much and my D8 is happy we are separated b/c she sees him more now she says than she used to when he lived with us (spends 10 hours a week with them now).
I think I have a different perspective b/c I was raised by my widower dad (mom died when I was 4 and was in the hospital a bit before that). I think my dad did a good job raising me and my brother. It is sad that maybe our kids have only 1.5 parents b/c the WAS is not very good at parenting, and they realize that they have one parent who is stable and will always be there, but I wonder if it's ok if they know they have that one stable parent to have the kind of absent parent, and maybe that's ok. I encourage my H to spend time with the kids and call them everyday and etc. but I don't plan on agonizing over my H not taking these opportunities when available. I think that's one of those things where we have to realize we don't have much control over our WAS' parenting, just ours, so focus on making ourselves the best parent we can be? (And take my kids to a C of course!) Karen