An admiration of femininity is just thinking a woman is beautiful. Nothing wrong with that. But staring at her breasts, mentally undressing her, imagining sɛҳuąƖ relations, etc. is lust.
There are many degrees in between that too, from just a purely aesthetic appreciation of her beauty, to a nearly-subconscious sense of attraction, to a stronger / explicit sense of attraction, to outright lust. And each step of the way there can be degrees of consent from the will (in which the sin consists). But the slippery slope into impurity starts with giving in at the first stages. At those first stages, we're generally not speaking about mortal sin (or even sin at all). But you might start letting yourself feel some of these things, and then it escalates into the next phase. So the subsequent temptation is a direct result of giving it some freedom in those earlier stages. All this assumes that someone in general intends to avoid grave sins against purity. Otherwise, they give in completely at every stage. That's why venial sins very often slide right into mortal sins. By committing venial sin in the early stages, you're setting yourself up for greater temptations with each subsequent stage ... and are creating the habit of consenting to sin in general. Pilot gave in to non-mortally injuring Our Lord (venial sin), with the intention that it would avoid having to kill Him (mortal sin). He hoped that by doing the venial he could avoid the mortal. But it doesn't work out that way.
So, for instance, you said that there was nothing wrong with considering a woman beautiful. Even those thoughts, if they arise on their own, we need to turn away from quickly. Acknowledge it and move on. It's very easy to fall into the trap of: hey, there's nothing wrong with admiring her beauty. Next thing you know, though, a sense of attraction is there. Then you might say, well, this isn't sin, as it's just natural. Next thing you know, you're sliding into at least venial sin.