Yes, this is BL. Pete (Tay Tawan) and Kao (Newwie) were also in the prequels "Kiss the Series" (2016) and "Kiss Me Again" (2018). The focus of the prequels were the 5 San- sisters and their respective love lives. Sandee (Mild Wiraporn), one of the sisters, is Pete and Kao's friend, and she and her sister Sanrak (Mook Worranit) were the lead characters in KTS. Their partners were Thada (Mek Jirakit) and Na (Tao Phiangpor), respectively.
Their older sisters Sansuay (Pango), Sanson (Thanaerng) and Sanwan (Fon Sananthachat) were the leads in KMA. It's technically a prequel to KTS. Their partners there were So (Earth), Matt (Mond Thanuchai) and R (March Chutavuth), respectively. Pete and Kao were also considered lead characters in KMA, and GMMTV has devoted several clips just showing their scenes for the entire series.
In DBK, it's now Pete and Kao's turn, as well as the other couple Mork (Fluke Gawin) and Sun (Poss Plapodd). Mork and Sun were introduced in KMA. DBK is the sequel of KTS. In KMA, that's where Pete and Kao met and got together. In KTS, they were already a couple but they decided to hide it from their friends Sandee, Thada and June (Nicky Nachat), until they were forced to reveal their relationship to them.