Well Len has the advantage of being young. The female cast finds him adorable instead of finding him a perv. Probably helps that a lot of times he's portrayed as being one of the more level-headed people in the main "cast".
Gakupo is older, and doesn't really help himself by being absurdly pretty (sometimes to the point of creepiness, depending on the artist). Plus Luka (his most common pairing by far, from my experience) acts very tsundere towards him.
