Why don't men take single women with kids seriously?

I was married for 14 years, which provided me with 3 wonderful kids. I divorced my husband 4 years ago because I upheld my wedding vows and he didn't. I am 40 years old and have had 3 separate year long relationships with some wonderful men, none of which have children of their own.The problem comes up that with all of these men...they tell me that they understand what it takes to be in a relationship with me and that they understand how important it is for my kids to be a part of their lives. I don't even introduce someone to my kids for months until I feel secure in how the man feels about me. BUT each time with each of these men, they began talking marriage and when I agreed to start moving towards that, every one of them freaked out when the reality sets in that they will be taking on me AND my kids. The last man that has been in my life is the most wonderful man ever and I really thought that he was going to be the one that would love me and my kids and we were discussing marriage but just last night...he told me that he didn't think that he could do it with the kids.

WHAT THA'? Why does this keep happening? It's not like I hide the fact that I come with kids and the minute I let my guard down and look to creating a life with someone, they run.

Help me understand.