That a person does not have interest in having sex themself doesn't necessarily say anything about what they think of someone else having sex.

It really doesn't, but social attitudes toward sex are so royally effed up that it's hard at times for allos and aces alike to separate sex from ideas of virtue/vice.

You are far from alone in seeing the parallels between aphobic and biphobic rhetoric--you may have seen this post or ones like it on Tumblr.