So... I am dating this guy for a couple of months now, and it starts to look serious.
The issue I am uneasy about that he has an ex, who he had an on-and-off fwb relationship with after their divorce. He told me about this yesterday in a very offhand manner, when we discussed a "sex with ex" situation in a couple we both know. Like: "Yeah, me and ex-Mrs Right used to see each other for some easy fun every time we both are single". I probed further and apparently the last time was just a couple of days before we went on our first official date (but already were heavily texting /flirting). 
They have children together so there is plenty of contact between them, and he stayed at her house a couple of times when there were illnesses or emergencies. Am I wrong to be so jealous and insecure now? Or is it a normal arrangement and I am overthinking this?