Why wont he spend the night?

my husband and i have been divorced for over a year, but have still been hooking up. he has spent the night with me a few times, but refuses when i ask him to come stay again. why? he says it may be emotional. i dont understand. i try to be “cool” about it and i definately dont bring up the past. what could be more emotional than having sex with your ex? its just a bed / sleep thing right? what am i not seeing?