ahaha Ah My Goddess... yes its about their relationship, but AMG its a small victory for Keiichi to even give her a present or eat her cooking because of all the wacky antics that pop up mixed with Keiichi's inability to say "No! STFU!" But there's always been this air that Kei & Belldandy are this couple that's been married for years without actually taking that step. I kept up with it for a lonnnnng time, but eventually the part of me that though Keiichi was a sweet sensitive guy turned into "Jeezus man D:< Belldandy wants the D, go tie the knot and take a long honeymoon for her sake and kick Urd & Skuld to the curb" and i just didn't have much interest in keeping up with the manga and sold all my copies to some guy in Wisconsin.
but anyway, like you said, it depends on the nature of the show. I think it also depends on the audience
In manga for young & adolescent girls, love is still something that's secret and innocent about. For example, in Kimi ni Todoke it takes 11 books for the couple to start dating, and 17 books for them to kiss. not even make out. kiss.
Meanwhile manga for grown women - if there's not sex in the 1st chapter, its somewhere in the 1st or 2nd volume. (Hataraki Man, Paradise Kiss, Butterflies Flowers, for example)
I think the same could be said for shonen & seinen manga - but Shonen-type stories the romance is such a second thought to it, that it ends up being endgame because "oh yeah, I guess Kimiko-chan should get with Shunsuke-kun now that the enemy's dead"