The difference is that ecchi does not show the genitals (the vagina and the penis). Hentai does, it doesn't matter if there's sex scenes or not. There are plenty of shows with sex scenes that aren't ecchi.
Sometimes there are some occasions where they do show a penis but not like in hentai. It's usually censored in a strange way, rainbow beams Koe de Oshigoto IIRC or simply drawn in silhouette without detail. Many times usually for humor like in Katte ni Kaizou. Vibrators do not need to be censored but are in some shows such as Seitokai Yakuindomo but not censored in Girl's High.
Whether or not there is sticky white stuff does not matter. Aki Sora has plenty and Kagaku has a load of lactation, both are still ecchi.