#hmmm....

In a game where we have canon same-sex couples.

In a game that is very LGBTQ+ friendly.

Kris, Frisk and Chara are not your self-inserts. They're their own people and they are referred with they/them pronouns. Not he/him. Not she/her. Not any other pronouns. But they/them.

Mad mew mew and Metatton are implied trans, as they went from sexless to female/male (Metatton - sexless to male, Mad mew mew - sexless to female)

Alphys is implied bisexual. Has a crush on asgore and Undyne, and in the end of The True Pacifist Route, Undyne and Alphys are dating.

And many other characters that are implied to be LGBTQ+ are not yours to deny their sexuality or gender. If you can't stand LGBTQ+ to the point of quitting the game because of it, then quit undertale. Don't force your homophobic and transphobic beliefs on characters that are canonically not what you want them to be.

Edit: I'd like to add a few more facts (now regarding the utmv community...)

Cross is trans-coded.

Dream and Nightmare were born sexless who present masculine. They weren:'t born as either male or female but sexless.

Fellswap sans in his older sheet is said to have issues with his gender identity.

There are sanses that go by they/them pronouns.

Ink is canonically aroace and uses he/they pronouns.