Is it haram for an unmarried girl to think about having sex? By this I mean seeing herself having sex with a guy who she finds attractive and thinking about how it would feel if the were a married couple in bed.
Sexual fantasies, I would say, are not uncommon for Muslims. The only problem, for men especially, is that sexual fantasies may lead to something more, including masturbation, or even worse, committing zina. That being said however, having sexual fantasies is by no means a sin, as long as the person does not act upon it.
Therefore, it is very important not to act upon a fantasy, whichever form it may be. So as long as the person is in control of the situation, it is not a sin.
Finally, take a look at this site that answers in more detail a very similar, if not the same, topic.
Thinking about haram act till has not been done,Also till intention wouldn't be Sexual satisfaction and be without Manipulation,isn't sin.
But pay attention It leads to sin and makes injuries in person 's mind and make it dark.It 's better to avoid thinking about that.(ayatollah Makarem)
Hazrat masih(Christ) (a.s.) said to his Followers:
So It 's better to avoid thinking about sins which leads to sin.
First of all, if the thoughts are un-intentional, or in dream, then its not your fault or not a sin at all. But if you intentionally visualize anything like that, then it should be avoided completely, as it can be first way towards Zina (as committing Zina's first step starts from thinking about it, which fuels your lust). So you should avoid anything, which fuels the lust.
If you want to visualizer about it that how it would feel, with intention to know that situation, its my personal opinion to avoid that too (unless you are 100% sure that you have control over yourself completely).
(Note: my opinions are based on very limited knowledged i have about islam, and i am not a scholar, i would recommend that you consult a scholar as well, in this case)
And Allah knows the best.