I think sitting on your partner's lap in public is too FAR... especially when there are plenty of chairs around!!!!
I do admit that sometime I sit on my husband's lap but usually when there are not enough chairs for everybody (friends) so I usually gave my seat up for a person who doesn't want to SIT on somebody ;-)
Holding hands, hugging, and pecking lips, for me, are minor forms of PDA that can be tolerant. if you keep kissing for longer than two minutes, then .... yes it is too far. But if you peck for "hello" and then "good bye" then I don't see how could that be disgusting...
My husband and I usually tease each other by groping eachother's butts in public, but it was all in tomfoolery-- not exactly an act of affection.
As teachers, my husband and I do draw lines on PDAs in front of students in Deaf events. He refuses to kiss me but we can hug. that's it. Ha. Whatever. What's the difference between students and the public???? WHat if one day, the student happens to be in a general public and my husband kisses me-- will that student gouge eyes out?? no. I don't understand my husband's logics for no PDAs in front of students outside of classrooms.
(In classroom settings, we are TOTALLY hands off. we only sneak in a peck of kiss if we are all alone in mailroom or something before we leave to our separate classrooms)