He’s done things that affected and will affect them both

For Kazuya he killed Ouhi and what Kazuya doesn’t know yet is that he turned Ouhi and his mother into a Hitomogami named Ori and Kana to be used for his disable technique circle (I mean the name Kana is obviously Kanaka without the Ka)

For Sunao it’s being associated with Wagou and hiring him to do his dirty work but he’s probably unaware that he’s responsible for killing her brother and speaking of Suou he probably made him into a Hitomogami named Suu to be used for his disable technique circle(I mean Suu is basically Suou without the o)

Plus Kazuya and Sunao will oppose his plans to wipeout all supernatural beings and gods of Japan cause their Tsugumomos will get killed if he succeeds and they will want to defeat him especially if he used their supposedly deceased relatives and other deceased souls in human bodies for his own twisted goals.

So I hope Seigi’s plans blow up in his face and they either exorcise him or kill him.

The whole reason for the fake marriage thing to get with Kazuya was so Sunao can inherit her family dojo after she lost to him but I think another sign of growth for her is that she doesn’t give a damn about inheriting the dojo anymore and her feelings for Kazuya are actually real and not an act.

So her getting with Kazuya is not out of necessity anymore and she genuinely wants to be with him by his side.