If Seju wants to see reactions from Sumin, she can just tell Sumin that she is planning on getting marriage with her potential marriage partner. It would be better than sleeping with Nami, right?
I believe that Sumin isn't really taking the threat of the marriage partners seriously because Seju doesn't really seem interested in them. So she doesn't really feel scared that she might lose Seju to a marriage partner. Nami has been dropping a lot of hints to Sumin about her interest in Seju though. So if Nami and Seju sleeps together, it might indicate to Sumin that there is a huge chance that she could be losing Seju for reals for someone who can actually take care of her.