When is it crossing the line?

So I'm in a new relationship and I knew from the get-go that he was very flirty with other women but he swore that once he's in a committed relationship he stops. I'm a paranoid person so if I get a bad feeling about something I'll do some digging to see what I can find, and something told me to check this cosplayer's insta (all she posts is lewd stuff, to promote her onlyfans I suppose) and saw him flirting with her in the comments! She's married and lives hours away but it bothers me that he's doing this, especially since he actually knows her. I'm the type to let problems like this build up until I can't hold them in anymore so how do I nip this in the bud? Do I ask him about it at the risk of seeming crazy or do I try and let it go? If you think the latter is the correct option then could you please provide some actual advice on how to deal with jealously? Please don't be mean.
When is it crossing the line?
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