It really was insensitive.
Mamoru's in the same position as Saki in the Saki-Shun-Satoru relationship, except Saki's got her comfort pillow ala Maria, while Mamoru's got his drawing board. Maybe it was out of sheer curiosity on Saki's part since she doesn't see Mamoru having some sort of backup partner.
Still, after Saki said that I thought "Do you srsly want him to murder you or something?"