As Ptak writes,
There's plenty of heads smashed together that don't leave a lot of room for faces, and in general it might be the sensitivities of the Golden Age's age that cast away the kiss itself.
It's kind of strange to go through the trouble of illustrating kissing without illustrating the kiss; the effect is much like the "fade to black" technique some old movies employed. But chances are, no one was reading these comics for the makeout scenes. And captions like "But when you tried to beat my brains out — I knew I loved you, baby!" aren't exactly romantic.
But you know how when you can't see the full scope of a situation, your imagination runs away with you? Chances are, not seeing lips touch could ignite weird and vivid visions of what these couples are actually doing. Although: Young readers interested in adventure and intrigue probably just skipped the smooching anyway; Ptak notes of the constantly obstructed faces: "Perhaps the kids never really noticed this, rushing to get ahead to the end of the confusing romantic interests."