A fatwa by Shaykh ‘Uthaymeen my Allah have mercy on him says
Praise be to Allaah.
Undoubtedly this man who kissed a girl who was a stranger (non-mahram) to him has not understood the wisdom and purpose behind fasting, because this man has committed an act of falsehood. The Messenger (peace and blessings of Allaah be upon him) said: “Whoever does not give up speaking falsehood and acting accordingly, and ignorance, Allaah has no need of him giving up his food and drink.” If he did that and forced her to do that, then in this case it was an act of both falsehood and ignorance, and he has failed to understand the wisdom and purpose behind fasting and his reward is diminished without a doubt, but according to the majority of scholars his fast is not invalidated in the sense that we do not say that he has to make it up.
Fataawa al-Shaykh Ibn ‘Uthaymeen, vol. 1, p. 516.