Alec is comfortable with facts and figures. He'd much rather read about killing demons then actually go out and kill them. It's not that he is afraid (not really, anyway) - he just isn't a warrior. In a battle, he can tell you, without hesitation, which kind of demon it is, its diet, its weaknesses, and how hard of a blow it would be needed to kill it. He just won't be the one to deliver that blow.
Alec was always more comfortable with facts and figures. For instance, he knows that one male Shadowhunter plus one female Shadowhunter equals good. It equals a happy Clave and lots of Shadowhunter babies.
Now, one male Shadowhunter brother and one female Shadowhunter sister? That equals bad. That equals not a happy Clave and disgust from the people surrounding and lots of Shadowhunter babies with arms growing out of their heads.
One male Shadowhunter and one female Downworlder (or vice versa)? Well, it depends on the Downworlder, he supposes. Isabelle did 'date' that one faerie, but that didn't exactly last long.
The last equation is the one Alec fears the most. One male Shadowhunter and one male Downworlder? (He tries not to be too specific. Tries to think of it as an equation that could be applied to anyone. Tries not to think of it as one male Lightwood Shadowhunter and one male warlock who also happened to be the High Warlock of Brooklyn.) That was disastrous. That equals an angry Clave and angry Shadowhunters and being stripped of your marks and becoming a mundie. That equals the end of life as he (er... they) knew it.
With that thought in mind, Alec wonders why it is he still finds himself outside Magnus's front door so often.