Before starting consider sikhism as a way of life rather then a "Religion".

I will put it in this way:

1. When we call somebody a "Sikh" that means he/she has understood sikhism and practising the same.

2. This means his/her thin king and way of life will be greatly (Infact Totally) influced by sikhism.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/love-and-marriage/8803-is-sin-sikh-marry-non-sikh.html

3. Now he/she will marry only that person who got the similar thinking.

Now in practical life when we call some body a sikh then it purely a relative term :

What I mean is to what extent u follow sikhism is idealy some thing "Discrete" but preactically it is "Analog" for most of the people.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=8803

so people follow sikhism to different extents ( Rather to the extent where they feel comfortable ) But not all of them !

In summary : As per what i know "It is not a SIN in sikhism to marry a non sikh but actually a true sikh will not marry a non sikh."