So presumably Luke is mellow and sensible enough to lay off the same kind of "Jedi can't have intimate relationships of any kind because having a girlfriend will make you kill-crazy" rhetoric that screwed up his dad's life so badly, whether or not he knows how that all played out -- just because he's a genuinely good dude and he has faith that his students can discuss that kind of thing and its ins and outs without just receiving an ultimatum.

Somehow this turns into Ben interrogating his teacher about his romantic attachments. Mostly it's stuff about friends he had a thing for before pursuing his life as a Jedi (those deleted scenes with Biggs, anyone?)... and then the topic of Leia and/or Han comes up, and Kylo is real weird about it.

(Alternately, this same discussion happening after Ben's heel-face turn, with him actively trying to use "well, you wanted to fuck my mom, didn't you? what's the problem?" to hurt his former mentor.)