It's "sitting in a tree" here.
The syllables wouldn't work right with "under".
The five syllables of sitting in a tree match with the SSING of kissing.
First comes love, then comes marriage, then comes _____________ with a baby carriage (you repeat one of the people's names because they're the parent).
Not that it matters.
Yall have never kissed in a tree?
"In" is better auditorily. But "under" is better semantically. Its hard to smooch when you're trying to stay balanced on a tree limb. And it's "dont sit