Agree. Not a good sign. A girl who loves you, is ready for an LTR and worthy of an LTR will not even want to do this.They all have a few red flags. However, you asked her not to go out drinking with a person who has sexual interest in her, she went despite your asking her not to, there by demonstrating her sexual interest in him. Not only a red flag, but a huge deal breaker. She will pull this shit in the future on you.
They will try to play dumb - call you 'controlling', say 'don't be silly, he doesn't like me!' or 'I'm not interested in him!'. Sometimes I really wonder if they believe their own BS.
Just don't be needy, domineering or angry about it. Calming state something like 'If you want go drinking with other men who are sexually interested in you without me, then I don't think we're ready for an LTR', 'I don't think we'd work out'.
Hidden secret - she actually wants you to do this to her, subconsciously. For you to desire her in this way. Previously I had been like 'yeah, sure, have a fun time!' (to be all non-needy) and found out this actually hurts good, honest, genuine women. As Waverer says, they see it as the problem that it is.