Yes, some aparents (and people in general) can feel the presence of a birthparent can be threatening to the adoptive family.
here are some links to some current threads that discuss some of these feelings
A little bit of honesty...the word "Mom".
Here is my truth..what's yours
For me, I know if our situation was closed I'd feel VERY threatened by the idea of a birthmom. I know because when we first looked into adoption i wanted to be in a closed international situation. I didn't like (and frankly still don't care for) the idea of another "mom". I worried that we would be treated as babysitters by birthfamily.
Now that I am IN an open adoption - I don't feel threatened by his birthmom. I really like her. Her presence has actually helped to ease my fears as opposed to increasing my anxieties.
There are times I read things on here and think - omg, that birthmom would freak the crap out of me.....so I'm not saying that all aparents who have fears about birthparents are over reacting.
and I even understand how someone can want a closed adoption - and still not feel threatened by a birthparent - just that they feel it is best for their family.
I'm not sure how much I helped...but do check outthose threads.