Referring to KuroeNekumi's comment, I suspect the exchange on page 19 is a reference to Japanese consent laws/statutes. While the base statutory rape law sets the age of consent at 13, prefectural statutes commonly raise this to 16-18. However, these statutes typically contain an exception for "sincere romantic relationships."
So, the logic behind the exchange here would be something like this:
"Both parties have to (be able to) consent before they..."
Haru, not being at the age of consent: "But there's no problem if we're lovers (in a sincere romantic relationship)."
But not having seen the original text, this is all conjecture. Maybe this manga is just weird and rapey, who knows.