It's not fun when you're horny but you can't really broach the subject. When there are reasons - you're at your parents' house (even if they are out); you're not sure if it will be particularly well-received or not; your girlfriend has been given the Hannibal box set for Christmas and you're also wanting to watch it.
If you've had an odd uncomfortable, sinking feeling in your stomach ever since Christmas finished, but you still want to have sex. If you're not even sure if you can, you suddenly feel like you may not even have the ability. That's not fun.
There may even be a moment when you manage to make it into bed with your girlfriend, when she's naked (for the first time in what feels like ever), and she gives indications that she would, in fact, really like to have sex, but you've just had a complicated and over-detailed plan about bits of the immediate future and the amount of thoughts in your head just won't allow you to let go and enjoy yourself. Yeah, that's not fun either.
That's really not fun.
When she starts to pleasure herself, however, when she's incredibly wet and didn't even realise it. When she's shuddering and gasping and making sounds that distract you, that turn you on. When, despite all indications to the contrary, you find yourself on top of her, actually having sex after what does, indeed, seem like forever (and has in actuality been more than a week, maybe two - which still seems like a long time)...
...yeah, that is fun.