I think it's more complex... porn actress don't seem to consider anything tabu... In a way, this mean freedom and free mind... When sex it's everyday with anybody... everything it's normal! I don't know about their fetish...
Fetish is something that excites you and it's not necessarily a tabu!?
and what I found on wiky : "Touching lips produce nerve stimulation and brain releases oxytocin, dopamine and adrenaline into the bloodstream which leads to some positive health effects. The release of oxytocin produces sweat and erection penis or clitoris. Dopamine produces adrenaline feeling well and increase tone, blood sugar, blood pressure and heart rate"
We all love kissing the person we find attractive... it excites us more than some other things we do... but it can be strange to find attractive an old man... why old men can't be attractive and sexy? we all get old...
Sexy is an attitude!