my (22m) girlfriend (18f) told me last night that her ex had texted her that day and started talking about sex. she told me this as if it was funny and i was supposed to laugh at the conversation they were having. of course i didn't, and stopped giving her much attention until she changed the subject. but i didn't even know she still talked to him. i'm not really okay with that, but how can i tell her that without sounding jealous and insecure?

i may break up with her or i may not, but i am asking for advice assuming that i will continue a relationship with her. so please don't just tell me i need to breakup with her. thank you!