Warning flags are one way Ancestry.com tries to help users reduce the number of errors their files. Apparently the parameters are not set too closely when the date of birth is approximate. I was able to enter a 1780 marriage for an individual born about 1845 without any warning flags or error prompts appearing. When the year of birth was changed to exactly 1845, there was a warning and I had to acknowledge the inconsistency of what I was trying to enter. It would seem that there should be a warning when a marriage year of 1780 is entered for someone born in approximately 1845. This screen capture was made on 8 March 2016.
See our followup on what does happen if the year of birth is not approximate.