"Summer House" alum Luke Gulbranson is a big believer in guy code and girl code ... telling us he doesn't approve of the way West Wilson and Amanda Batula went about getting together.

Luke joined us on the "TMZ Podcast" and we couldn't let him go without asking him about the messy love triangle.

As you know ... West used to date Amanda's costar and close friend, Ciara Miller.

Luke says Amanda and West are breaking the unwritten rules and he says they've shown a shocking lack of loyalty.

The way Luke see it ... Amanda and West should have gone to Ciara first about wanting to start dating each other instead of sneaking around and finally having it out in the open.

And out in the open it shall be as the much-anticipated reunion filmed Thursday... and it seems like everyone's cards will be on the table.