הוכח תוכיח את עמיתך, rebuking a person who is doing a sin.
[Vayikra 19:17] "You shall surely rebuke your fellow, but you shall not bear a sin on his account."
[See Rambam (Sefer Hamitzvos, Assei, 205) "The 205th mitzvah is that we are commanded to admonish a person who is performing a transgression or who is preparing to do so. One must verbally warn him and admonish him..."]
In this case, by being an active participant in a Conservative/Reform ceremony, by serving as a witness on their Ketubah, is the not only the antithesis of rebuke, but also sends the message that we tacitly condone their deviation(s) from accepted Halachic practice.
[Note: This answer draws inspiration from the Sefer Chafetz Chaim (Essin, 5, Be'er Mayim Chaim) who formulates a similar ruling regarding someone who neglects to admonish a person who is beginning to speak Lashon Hara or Rechilus, and is an ample precedent for this Halachic approach].