Boyfriend won't share his Facebook password because "it's against the T&C", is that a stupid reason?

So, we've been together about 3 years, and while I know the PIN to his phone, I don't know his FB password (or IG or twitter) and he doesn't like me reading his mesasges.

I sat him down and told him, calmly, that it's important to me we share passwords because I feel it shows trust, but he refused, stating that it's against T&C, and that he won't be sharing. I feel this is just an excuse, am I right?
Yes, it's a stupid reason, he should give you the passwords
No, it's a valid reason.
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Boyfriend won't share his Facebook password because "it's against the T&C", is that a stupid reason?
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