Your question is very vague so my answer may not be all that helpful. But as it relates to you undermining him - I assume he told the kids one thing and then you told them something different within minutes? I was guilty of that too for a long time - and I still have to bite my tongue. Our kids are 14 & 17 so it's been a long time and lots of effort. My husband is more lenient than I am in most situations but on a few he's much more strict. So there have been times he became the "disney-dad" and let them stay home from school (if I was out of town with family or business and he had the day off work) and they'd go to a movie or the toy store (meanwhile we do laundry or grocery shopping on my day off...!) or times when I'd be trying to discipline or tell him the kids to do chores and he'd say - "it's OK they don't have to _____" . It would make me crazy! or I'd do something similar when he's begin barking orders or ignoring the kids so he could watch a game and I'd take the remote, pause the game and tell him to pay attention to his kids.
In all of these situations we send this mixed message to our kids that the other parent isn't neccessarily right or an authority. So the message the kids get is "dad's word isn't final so I don't have to listen to him" or "mom told me to do something but dad thinks it's not important so I don't really have to do it". So while they're little kids it's about snacks and toys and sharing. When they're teens it's about schoolwork, drugs, sex and driving responsibilities - more life and death stuff. But the patterns are set in the early days and if our kids don't know to listen to us when they're young they won't do so when they're teens. And while I'm still guilty of it, as is my husband we have gotten much, much better over the years. But it does require sacrifice of our will (maybe we're not always right?, maybe it's not worth it to make my husband look like he's not the authority?)
We had a long talk one day while out on date-nite over sushi about the need to present a united front when in the presence of the kids. All disagreements about the kids must be done in private. Now if there's something really important that I want the kids to do I will talk to my husband ahead of time and make sure he understands why it's important and to ask that he not make contrary statements - but instead will agree and support me. For example, I really wanted my 14 yr old son to go to sleep away camp for a week. Many of his friends were going, I knew he'd like it once he got there and it would be good for his overall development. So before I talked to my son about it I talked to my husband. Initially he didn't think it was so vital that he go, but once I explained why I wanted him to go and why I needed my husband's agreement on this - he agreed. My son leaves for camp tomorrow morning!
If he or I says something to the kids that the other doesn't neccessarily agree with we will now get the other person aside (even if we have to send a text) and express our difference of opinion. It is a much better way to approach raising kids. Most of the time once given the opportunity to understand the other's position we will agree with eachother.
Now - I think there is a concern that your man is willing to threaten to end the relationship with an ultimatum over this without serious conversation before. Part of the problem with having kids before the marriage comes along is that lack of rock solid committment. It happens inside a marriage too - but it's so much easier to bail on a living together & sharing kids than it is to bail on a marraige. Before we had kids and we were living together I gave my now-husband the marraige ultimatum - and it was more from a legal perspective - about finances, will, etc. i explained how if he died I would be legally entitled to NOTHING, that I couldn't make decision about his medical care if he was seriously injured, etc. After we got married, bought a house and had kids we went through many difficult times (I think having small children is very tough on marriages). And finally after many threats of divorce I told my husband to go ahead and leave because I didn't want to live under the threat of him leaving me with 2 kids to raise. I told him I'd rather just get started on that life if that was going to eventually happen instead of worrying about it. When he backed off and agreed not to leave and to work on our "stuff" I told him that if he wanted this to work he had to promise to never again bring up divorce as an option. And he hasn't. We've worked on our stuff, God did some awesome (and really difficult) things in our life and our marriage is stronger than it's ever been. We are going on 18 years of marriage - we've been tgether for more than 20! Who would have thought...
Anyway - the two of you have to sit down and be willing to not always be right. You ahve to realize that not every disagreement is a battle to death over your perspective. 90% - 95% of the time it's stuff you can resolve to have the same position on in front of the kids. And PS - it's a good thing to show that same united front and loyalty to eachother to your circle of friends too. Women look for love from a relationship, men seek respect. If your man gets your respect he'll never look elsewhere.
Good luck mama!