My ex is dating someone new and the kids like them

Not sure why this bothers me as much as it does. Their mom's been seeing this guy for like 8 months now and the kids talk about him constantly. He seems fine, probably is fine, but it just feels weird. I know I shouldn't care who she dates but when my 8 year old asked if he could sleep over at moms during my week I had a moment. Am I being unreasonable? I'm remarried too so it's not like I'm trying to get back together or anything.