Female Frisk

well, the answer could be "because this is internet and here we can do whatever we want"

but in my case, i think frisk looks like a character with an ambiguous gender, that's could be the cause of the sexualization of he/her.

i draw her in a sexy style just for add a little of sex appeal, AND because i'm really bad for draw male characters.

i hope you have a nice week

Nope. Toby Fox intentionally left Frisk genderless, and characters in the game even use they/them pronouns for Frisk. Toby did say the gender was up for interpretation, though, but many like to use the fact that Frisk is genderless as evidence of them canonically being agender/non-binary.

Brock: *Sees Female Frisk starts to fall inlove* A hot girl!!... You're so beautiful and hot