Also, Marianne's main objections to Colonel Brandon are:

--he's quite a bit older than she is
--he's already been in love with someone, and you can only fall in love once in your life
--he didn't have the dashing looks and behaviour she expected to find in her true love.

These are not real reasons to object to someone as a marriage partner--they are very superficial and immature objections, and not on the same scale as objecting to actual character defects in a person such as being vain, self-important, and silly, like Mr. Collins--or, as Marianne learns about Willoughby--a liar, a cheat, and a scoundrel. It's not wrong to object to a person who is likely to make you miserable and whose defects are not something that is ever likely to change.
Marianne's romantic ideals were dangerous because in insisting on the superficial, she was not at first looking deeper for qualities like honourability, kindness, sincere affection, intellgence, loyalty, etc. or the fact that Colonel Brandon was actually very like her in temperament. Marianne was objecting to things that really make no sense to object to: Why should it matter, for example, if a person had already been in love before and that didn't work out? Why should it be assumed that an age difference is an insurmountable obstacle or that anyone over the age of 27 is incapable of having feelings? Why should it be assumed that only someone who is exceptionally good-looking is capable of having true feelings?

Those kind of romantic ideals are dangerous because they are limiting, ignorant and unfair.