To the final question of the OP - so long as the discussion remains respectful, there is no reason why you shouldn’t try to explain why those things are sinful. Whether or not that is possible depends pretty much entirely on the dynamic between you and your mother (and I’m not saying that if it is not, then your dynamic is bad or anything like that - only that some personalities and relationships together would make such a discussion between a parent and child fruitless, and some would not).
As to how to explain it - perhaps it would be best to point that there is a distinction between noticing and admiring beauty in others (fine and good) and using others as objects for one’s own pleasure (lust, bad). The problem with lust is that it takes a human being and reduces them to less than human. It treats them as a no more than a means to an end (sexual pleasure), a tool to be used - reducing a full fledged human, created in the image of God with a mind, reason, hopes, dreams, etc, to little more than a hammer that we want to use to accomplish something. And not even to accomplish something that is necessary, but only to give ourselves a passing pleasure - like a game boy, something to be played until we grow bored with it and then set aside until we want to be entertained again. Hopefully this will get the distinction across. I have spoken with people who didn’t realize that there was more to lust than noticing that a woman is pretty or similar, and I think pointing out that there is and what it is might be helpful.
As for it being “natural,” in the sense of “built in” - well, often so is the tendency to lie about wrongs we committed to avoid “trouble” (which really means “responsibility”), and any number of other bad things. This does not make them good. In fact, the doctrine of original sin tells us that we all have a built in tendency to sin, so merely having an in built tendency to do something does not make it ok to do so.
So if “natural” is used just to refer to the way things are, then it makes no difference whatsoever whether a desire to engage in a behavior is “natural”. Probably, in this sense, most or at least many sinful desires are natural to the person who has them, but clearly some actions are bad regardless of why we want to do them (lying as above, etc). Often, we say that the word natural would be better be said to mean something more like “according to reason” or “how things would be in a world without flaw” or similar - and using either of these definitions, we can see that neither lust nor masturbation would be natural, as treating humans as less than they are (treating others as objects in the first case, and treating, in some way, ourselves and possibly others as well as objects in the second) is definitely not reasonable and definitely not something that would happen in an ideal world.
