Sexual orientation doesn't just appear out of nowhere. You don't suddenly become bisexual, gay or asexual - so maybe look up your terms.
If your partner suddenly went off sex completely and said that never wanted it again, you can be 90% certain it's either medical or trauma. So you're basically asking if your partner is ill, would you leave them to get your end away?
Sure, if your partner had no plans to seek treatment then yeah, it wouldn't be a jackass thing to leave them. But otherwise I think it's selfish