Straight.com has a chat with Kevin Zegers (Alec Lightwood), they talk about Alec's sexuality and how that made the role more interesting to play.
"Mortal Instruments is a pretty big movie and the fact that the studio feels comfortable with one of their lead characters being gay is pretty indicative of peoples' opinion towards that sort of thing, which is that you don't really have to make a big deal out of it. It's just sort of is what it is. It's just one of the layers of the characters that I get to play."Read more over at Straight.com
In fact, he feels that the fact that Alec Lightwood is gay actually added more complexity to the role.
"For me, it makes the character more dynamic. There's a lot more to work with and his behaviour and what he does in the film is not defined at all by his sexual preference. In fact, he's probably one of the more bad-ass characters in the movies so it's certainly not something that we played or…it's not something that we needed to focus on. It's just one of the many traits he has."