You’re absolutely justified in feeling uncomfortable about your wife sleeping in her ex-husband’s bed when she visits the children at his home. It’s wrong for her to do that to you — but it also confuses the children. All children of divorce want their mother and father to get back together. It’s a fantasy they hold even when they are old enough to understand their parents have remarried other people. So when she sleeps in their father’s bad, even if he’s not in it, it sends a confusing message to the children that is not healthy for them.
If you trust her, then a perfect solution is that she sleeps on the couch! Or the sofa bed. Or a borrowed futon. Or even a sleeping bag. Depending on how often she visits and how long she stays when she visits, she could also stay in the kids’ bedrooms. And that’s just what’s in the house! There’s always a hotel or a neighbor who is willing to put her up there while she visits.
And, as jtrias2002 suggests, below, you could always travel with her to see your stepchildren. While that scenario may present awkward situations, there are many ways to modify it. You and your wife can stay in a hotel at night, and she can see the kids during the day. The kids may even want to stay in the hotel with you two, depending on their ages, because it’s a big treat for kids to stay in a hotel with room service and the hotel pool.
Regardless of what you two decide, the bottom line is that it is wrong for your wife to sleep in her ex-husband’s bed, and that there are many different adjustments she can make in order to still visit her children while sleeping elsewhere.