No its not a sin. Remember Paul said "I would rather have you as I" meaning not married, which can imply no sex. If you don't have the desire or don't perform any sexual activities than that is not a bad thing, God may have given that gift (which for some people that can be a true gift) so that you can do his will without being tempted by sex.
This is probably one of the biggest paradoxes between sex and Christianity. Lust and sex are bad. But it's also bad if you don't have babies, even though babies can only come from sex.
The Bible doesn't give very clear consensus about whether or not Asexuality is a sin or not. Beyond men having sex with other men, the Bible doesn't say much about sexualities at all. (Not that it really matters, because you 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.
Well Jesus praised the eunuchs "For there are eunuchs who were born that way, and there are eunuchs who have been made eunuchs by others--and there are those who choose to live like eunuchs for the sake of the kingdom of heaven. The one who can accept this should accept it."
Matthew 19:12 So clearly those who sustain from sexual relations or who are born that way are doing no wrong.
No it's not sin. Not being sexually interested in either gender is not a sin. The Bible states several times that there's nothing with it. I'm actually asexual, and I've never felt like God wanted me to change.
Well Asexual means having no sexual desires towards either gender basically so while that is both a good and bad thing that doesn't make it wrong.
If we're talking having no preference that's a different thing, but simply having no attraction, be it sexual or otherwise, seems to me to be more of a disability, not a sin and even if not a disability than simply a life choice because it doesn't violate the commands against homosexuality, and instead it promotes closeness towards God.
After all, one of the most important reasons to be in a relationship is so that you and your partner can bring each other closer to God.