It irks me when I start off a relationship with a woman and I make it clear from the jump that marriage is not in the cards, than years later the subject comes up. Pisses me off.
And I have no problem with couples having a relationship that is not intended to end in marriage so long as they don't have kids together. (I'm quite old fashioned when it comes to kids.) Those who already have kids who choose to remarry, when adoption is not an option, should have a good prenuptial agreement to protect those kids or other members of the family that must be considered. They might trust each other implicitly, but they can't always trust other opportunistic relatives to care about those kids or their inlaw's family.
My husband and I had our first date in July, we were engaged by September, and married by February. Because we were working together through most of our engagement and spent almost all of our spare time together the whole time, a seven month relationship was sufficient to know each other and work out most of the details. But we both wanted a soul mate and life partner so we went into the relationship with a common goal to achieve that and neither of us got cold feet or backed off the goal. (That was decades ago and we are still going strong.)
Often one senses or recognizes red flags that should not be ignored in the relationship or just realizes he or she is not yet ready for a lifetime commitment and either one of those situations is sufficient to end the relationship or at least cancel or postpone marriage plans. The serious stuff should be settled well before the marriage is finalized.
And HG, there comes a time when those who think they don't want marriage, a traditional family, etc. etc. change their minds about that. Or they fall in love and their priorities change. If it happens with both people in the relationship that is a wonderful thing. If not, one is annoyed and the other has his or her heart broken. It's the risk we take.