A magic user can use magic to do absolutely anything they can imagine, provided that they have the magical energy required to do so.
As promised, today I will be discussing one of the implications. So before you get too far into that sentence looking for something to discuss, I'm going to stop you at the third word. Those first three words there, "A magic user," mean a lot.
Specifically, you need to define who can even use magic in your story. And while to a degree I'm discussing just amongst your characters, I'm also discussing the world at large. Is it possible that everyone can use magic in your story? That's for you to decide. It's equally possible that only some people can. Or even that everyone can, but not everyone can use all kinds of it.
Even with that, you still need to decide who actually uses it (which in some cases, but not all, means who knows how to). For instance, in my current story-in-progress, Phoenix Mage, one type of magic user is a mage, which I define in the story as someone with magical energy as an essential part of them. All mages are, of course, capable of using magic. Not all of them, however, actually do. Most of them don't even know they are mages. And to show that you don't need to know how to use magic to actually do so, not all of my mages that do use magic know they're using magic of even have any control over it. Plenty of them use it on instinct without even noticing.
Furthermore, you don't need to limit yourself to one type of magic user. My story also defines another group of magic users called shamans as people who manipulate and use magic that is naturally found in the environment. A notable difference, however, is that logic itself will dictate that shamans MUST know that they use magic and they MUST know how to do so. Which of course doesn't even mention that you have to use magic to be a shaman.
This of course, leads to my next point. Once you've decided who can use magic and why they can use it, you need to let logic take it's course. If the only access to magic in your world is through having sex, you can expect that no one who can use magic in your world is a virgin. In fact, you'd probably expect them all to be sexual deviants who have sex at the drop of a dime (depending on how much they like magic and/or sex). The way someone accesses magic WILL affect the way they act. The sex magician is much more likely to try and seduce someone to get their way, for instance, than the magic user who gets their power from respecting nature or something like that.
This doesn't set hard and fast rules (unless your story calls for it), but it is something to keep in mind as far as suspension of disbelief goes. If the magic user who gets their power from nature sets fire to a forest, I expect a damn good reason for it, and so will your readers.
Got it? If so, good. If not, drop a comment and I can try to clarify. That's it for this week, I'll see you again in another couple weeks with Part III.