Is it weird my boyfriend has sleepovers at his bff which happens to be his ex gf?

My boyfriend was in a relationship with his ex girlfriend for 7 years. They left each other 10yrs ago and decided to stay friends. I've been with him 8m and noticed his weird behaviour. He speaks about her really often, has pics of her in his house and they even went on vacation together just before we met. Two years ago he asked her to get back together but then they decided not to.
He had a sleepover at her house and I got very upset. He was visiting the city for work where she lives and decided to stay at her place instead of driving home.
He did it a second time saying he didn't realise it makes me upset. He says it's normal and I'm the paranoid one bc they don't have an intimate relationship. He says she is now like a sister. He says society makes you think there is something bad about it.
He had at least 5 different options where to sleep that night: brother (5mins away), parents house (20mins away), my place (40mins away) hotel and to drive to his place (1 1/2hrs away).
He says I have to trust him and nothing intimate happens with her. They just like spending time together.
Am I wrong to think this is childish and wrong and disrespectful towards me?
Thanks
Is it weird my boyfriend has sleepovers at his bff which happens to be his ex gf?
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