We don't know for certain, but if we had to guess, we'd say that at some point in the recent past, Bollywood star Sunny Leone has performed a kissing scene with an actor who had really bad breath. We say that, because she used to do kissing scenes all the time. Ever since she made 'Ragini MMS 2' back in 2014, she's refused to do any at all. In fact, she feels so strongly about it that she has a 'no kissing' rule specifically written into all of her contracts, so directors don't try to force her into it.
The most curious thing about this is that Leone is primarily an 'adult' actress. That means she films a lot of scenes in which she's wearing very little, and goes far further than just kissing her on-screen partners. She has no issue with doing full-on love scenes in movies like 'One Night Stand' so long as she doesn't have to kiss her partner. Perhaps she just feels that kissing makes the scene too intimate?