I don't know why people are even debating whether they had sex. Zayn hasn't denied anything has he?
There are three options as to what could be happening.
1) He's guilty, and he has been advised by his managment to remain totally silent on it.
2) He was set up, and perhaps he will be taking the girl and The Sun to court?
3) He is not guilty, but has also been advised not to say anything because his managment believes that even if he talks about it, it could ruin really ruin 1D's Family orientated image.
Don't assume that just because he's stayed silent, he's automatically guilty.