It colors the team member's view of other people or other teams, whoever is being vented about. It's contagious and establishes a culture of complaining. It gives those vented to the notion that you might vent to others about them, which you probably do.
I have seen this take over many teams and it always starts from the top of the team.
I'd also add that venting to the team makes the team nervous, if you are saying X to them, what aren't you saying. Your job isn't to deceive the team, in fact if things are going bad you should be honest but show a path forward. Lying will cause people to lose their respect for you and hence stop following your lead. So you have to be honest, be humble but show the path forward to the team. And it is ok to not have a path at first, just be up front and give some options so they don't think things are at a loss.
I have seen this take over many teams and it always starts from the top of the team.