I don't think you need a reason for not marrying, unless you have bred children, then you better have a good reason for not marrying him or her in my opinion.

Anyway, if you are the marrying sort, I think the reasons not to marry are
1) you haven't met the right one
2) you met the right one but he/she is not available..i.e., married, lives too far away, too many other obligations etc.
3) right one, available, but has fatal flaws..gambler, drug user, alcoholic, abusive at times.

A subgroup of 1 is that you met a great person, but he/she does not have the elusive "it". Could have great character, personality, values, stability etc., but there is no point if "it" is not there.

I say otherwise quit dithering and get married already..

mg