The Gemara about Raban Gamliel is asking how he could look at her enough to take notice of her beauty. That is Histaklus, the Targum of Ma Esbonen El Besula. Normal, necessary looking, as in a conversation that is Mutar does not fall into this category. However, the Rosh’s son, Reb Yehuda writes that although this is the case one should refrain from all looking.
Al Tarbe has nothing to do with pleasure. Sam2 quoted a Nusach — which I believe is in Avos D’rav Nosson — that says Al Tarbe Sicha Im Isha Bashuk Afilu Im Ishto Mipnei Ma Yomru Habrios. And if you go Kepshuto, and not even Ishto Nida, then the Pshat is, like Toi quoted from the Chazon Ish, that it refers to what is outside and beyond what is called for in a normal relationship. It happens to weaken Shalom Bayis in many cases when the husband hangs around too much.
The Kol Bo brings a Medrash about a tour of Gehenom, with descriptions of punishments for those who caused men to look at what was not Tzenius.