Not a question, just very confused?

So my ex girlfriend of 7 years ago I have children with left me and started a new relationship, but when we see each other after a couple of days or when she picks up the kids we end up having sex. I don’t wanna say for certain I know why that’s still going on but I’d like to get some female perspective on why it keeps happening despite her being in a relationship and claiming she’s happy.
Not a question, just very confused?
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