Commentary for Kiddushin 131:5

The consequence of this one witness being believed is that she would be prohibited to him as an adulteress. She could not, however, be punished without two witnesses. As we saw above, punishments cannot be meted out with less than two witnesses. However, there might be consequences to one witness (I can just hear my parents say—“This is not a punishment, it’s a consequence).<bר>According to Abaye the witness is believed and the woman is now prohibited to her husband. Rava holds that since this is a sexual matter, we require two witnesses.