Having been down this particular road more times than I care to admit (3), I can offer you a word or two from the trenches.
I'm SURE my mother knew my first marriage would end up in the dumpster but she stood back and let me make my own mistake. I got married the first time for some of the most popular wrong reasons. a) All my friends were married; b) I was 27 (with 28 looming) and unmarried, which caused me to c) See all kinds of non-existent positive personality traits in my soon-to-be husband. Of course, after being married a short time I realized how many of his wonderful attributes were imagined. Adios #1.
XH-2 was an attempt to live my life the way everyone else said I should. Friends and family wished I would a) Seek out a college grad (I'm more the rugged guy lover); b) Go for someone who was an office type - suit & tie, soft hands, etc. (see 'a'); c) At least partially base my choice on earning potential (not generally a consideration for me); and finally d) Try someone older than me (THIS should have been my biggest indicator of disaster...). Five years, a costly divorce, bankruptcy later, I was on my own again. Kick #2 for a field goal and 3 points.
This brings me to #3. Thank GOD for my husband. (My 3rd and last.) It took some doing but I found someone who a) respects me, b) loves me, c) believes in me. Now most people don't have to test drive 3 spouses - and they are the lucky ones. If you can stand back and in your heart of hearts know that your daughter has made a responsible decision with just the right amount of passion and fun factored in, you can watch her life unfold with optimism. If you feel the relationship is "iffy" then you must let her travel whatever road awaits her.