I am childless, so take this with a grain of salt.
Although I am lucky, and both of my parents are still alive, they did get divorced when I was eight. The ink had not dried on the divorce papers, and my mother remarried. My father on the other hand took years to do it, and I met MANY a girlfriend. There was only one that I was a bit upset when they split, however I was a kid, and got over it rather quickly.
I think it's fine that you are going on this trip. I think your therapist is a bit of a prude. It's not like you met this guy at the bar last night and are going away together with your child. I for one think it's important to see how everyone gets along.
Bottom line, is your kids want you to be happy, just as much as you want them to be. I believe we have become a society that coddles our kids too much. Kids are more resilient than we give them credit for.