Of course. Why wouldn't I? I give zero fucks about this random guy's existence at this moment. He's just there. If my man and I are having a moment then we're having a moment and if it upsets this reason passerby to look at then he can look away.
I have no opinion of the random guy. He doesn't exist. The only man who exists in that moment is my boyfriend.
It's not to show off. It's not because I think this random guy is attractive. It's not because of any reason other than I want to kiss the man I love.
If I kissed my boyfriend in front of a random guy then the most likely reason for it would be that I just wanted to kiss my boyfriend, chances are I wasn't even paying attention to the random guy.
If it did have anything to do with the random guy however (which it probably wouldn't), it would probably be that me or my boyfriend thought he liked me or my boyfriend thought I liked him for whatever reason, so it was to reassure my boyfriend I only liked him or something.
A lot of couples are not phased by displaying public affection, they play tonsil tennis at an aquarium or in a supermarket, hey can have sex on park benches and in bus shelters regardless of what passers by think. Sometimes people do such things in attempt to make others jealous. Was this guy young and attractive?
Either she wanted to kiss her boyfriend and didn't care who was around. Or she wanted to make the random guy jealous by kissing her boyfriend in front of him - which means she likes the random guy and the poor boyfriend is being used.