She doesn't want to simply share wine and convo with the others because she wants Tav to herself. With the others it would simply be only wine/convo, nothing romantic like it is with Tav alone. She didn't want it to be a group hangout.
"Not tonight" suggest openness to the idea in the future. Think about it would you say "not tonight" to something you were dead set against? "excuse me, would you like to drink some sewer water"? "oh, not tonight" (said no one ever)
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A primary partner would still have to be shared. She says she would always want more than you'd have to spare, which means she wants your full attention.
Which I read to mean - I want the lion's share of attention. Which she does get in a poly relationship with Tav.
I agree with what @papercut_ninja and @Auric have said here
