She understands, and says yes!

"I see... And what would this tail?" Petra asked.

"I think you mean "entail?"" Dorothea asked, and Petra nodded. "Well, the Professor is pretty flexible with the terms of the harem arrangement. There are only a few things he would consider a hard rule."

"Like using Sothis's power to **** anyone into anything without their consent," Manuela added.

"Or trying to dominate Master, that's a no-no, he's not a submissive!" Sothis added. "Though of course, power play games between harem members are perfectly acceptable!"

"In short, you can dominate other girls in the harem, and I'm not against you dating anyone outside the harem," Byleth added to Petra. "I know you're something of a competitive switch, correct?"

"...indeed, Professor, I have very few problems with most forms of sexuality, and like to wrestle for top position," Petra noted. "In Brigid, we do not have the same baboos about sex as Fodlan..."

"Wrong word, it's "taboos,"" Manuela chimed in. Petra flushed, but continued.

"I am most intrigued by all of this..." she said. "I would very much like to explore this situation... Count me in on it!" she declared. "I suspect that I will be without regret about this..."

"Alright, then name any terms you might have..." Byleth nodded.