This is probably one of the biggest paradoxes between sex and Christianity. Lust and sex are bad. But it's also bad if u don't have babies, even though babies can only come from sex.
The Bible doesn't give very clear consensus about whether of not Asexuality is a sin of not. Beyond men having sex with other men, the Bible doesn't say much about sexualities at all. (Not that it really matters, because u shouldn't be basing all your morals based on one book anyway.)
I don't think any type of sexuality is a "sin", so long as you're not harming anyone with it.
EDIT: linken about Paul:
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