My interpretation was that, this gives Jun the assurace that Chungho loves him enough to tolerate how far he can go , the flashback was shown to indicate Jun's "Testing the water" moment
remember how Chungho didn't really wanna be a bottom? but he bottoms for Jun therefore Jun felt assured and he wanted to test until what limit can Chungho allow him until he gets told " No we can't do this"
If you recall, the first time they had sex is not because of the "mood" but rather because Chungho is anxious and try make Jun "forgive" him (he thought Jun was mad at him) and Jun, although he knows exactly why Chungho did that, goes along with it and even make Chungho submit to him by being the bottom
why does he keep getting flashbacks on their sex life? is it because he feels as if jun is controlling him or something?