yakno, i don't think undertale monsters approach gender and sexuality like we do, but I think I hit on something good when i proposed that toriel is so old that she still sees sexuality as something you do rather than something you are.

like she's got some stories from back in the day that will have you do a double take, but she thinks everyone did that so she pays it no mind. and I just imagine sans googling "how to tell your gf she's bi" but when he finally brings it up she looks confused for a moment before going "ah! yes, that. i am non-practicing now".

and you see the gears in his head turn for a good five seconds trying to decide what to respond, before finally hitting on "oh. cool." and dropping the subject altogether