He trusts her with his life. Why not a key to his house?

"Second, technically." César grows incredibly confused, and it's almost possible to see the equations and words flying through his head with that expression.

Then, he figures it out, and his cheeks redden, and he drops his voice to an embarrassed whisper. "... did I just proposition you? ... I-I mean someday, but I'm happy just letting the pace be whatever it is. You're worth the wait, and I honestly need it myself."