consider: mac and dennis being each other's first kiss back in high school bc "it's not gay we're just practicing, bro. practicing for the ladies"
this HAD to have happened at least ONCE. and i can totally see dennis being the one to initiate it by telling mac something along the lines of “look, we both know i’ve kissed more girls, so i’m the expert here. let me show you how it’s done.” and if he enjoys kissing mac more than any girl he’s ever kissed then that’s just a secret he’ll keep repressed until his 40s :~)