Almost every tradition had a practical element to them when they started. Both explanations could be true.

[Rin's self-loathing nature makes his haggard appearance all that more appealing. The scar on Tomura's lip catches her attention so thoroughly that before Tohsaka has a chance to think better of it, she kisses him there. Taking his lips in a lopsided embrace as she yearns to take it all his roughness and imperfections and draw them against her. Like accepting a punishment or self inflicting karma that is past its due.

It's brief, however. Because she can't shut up, or because she doesn't want to be tempted to use him like a weapon against herself when there is a specific goal in mind for this encounter? Either option could be the truth.]

What do you like? As your host I'm feeling... generous with my hospitality, should you need it.

[More like she'd rather show off and please her partner than focus on her own desires.]