The '90210' star was out grocery shopping in Malibu Thursday when he got asked about the women he's been spotted with recently, and ... he felt like venting.

Regarding his lunch date with Australian model Tina Louise ... he says he just happened to run into earlier in the day when we caught him out. Brian says Tina's "super nice," but gives a firm no when asked if he's dating her.

As for why ... it may have something to do with Courtney Stodden.

As we reported ... the 2 were seen hanging out in early June, but we were told it was just a collab on a business project. But, shortly after Brian and Tina were seen together, Courtney shared a vid of him and her hangin' in a hot tub ... a move that rubbed BAG the wrong way.

He says the video was from several weeks ago, and he was just trying to do something nice for her friend, Ashley. Brian calls what Courtney did "disappointing," and suggests it caused some problems for him and Tina.

Tough break, but the actor says he's not going to slam Stodden for what she did ... he's just not happy about it -- says she makes bad choices.