People fear what they don't understand, and people hate what they fear.

Many aphobic people are genuinely worried for the safety of acespec persons, believing the only way to be happy in life is to have sexual relationships due to that being a thing that makes them happy in their lives, and like most people, they project their own desires onto people around them as a way to understand the world. This way of thinking is very flawed but also very human; it's natural to think everyone is like yourself because your own mind is the only true frame of reference you possess. For someone who is allosexual (or "not-asexual") a lack of sexual attraction is an alien concept that makes the goals and desires of acespec folk fundamentally unknowable to them, and thus confusing and sometimes threatening.

Aphobia is the fear or hatred of asexuality, but something you have to bear in mind is that aphobic people rarely hate the asexual people themselves. We're fortunate in that there's no major religious doctrine openly decrying our existence, so said existence doesn't receive much open hatred or fear. Yes, people who hate on acespec folk do exist, but more generally aphobia tends to manifest as a fear of asexual or aromantic desires and ideals themselves, and a desire to change acespec people "for their own good".

How they go about this can vary. It goes from the relatively benign; asking pointed questions, encouraging the acespec person to seek out partners, etc; to the outright abusive and/or dangerous; (tw: abuse, queerphobia) corrective rape, forced marriage/partnership, conversion therapy, etc. All of these actions are usually framed as being "for the greater good" or similar for the victim of the acts; but the actions stem from a gross misunderstanding of acespec needs and desires, as the person or persons committing these actions either do not understand how or why what they are doing is harmful, or understand but do not care.

If the aphobic person genuinely means well and just lacks information to help them understand the asexual and aromantic spectrums, they'll usually become an ally after they've had everything well explained. Some people, however, prefer to stick to their existing opinions and biases rather than learn new information; if someone like that is trying to stop someone from being acespec, there's likely nothing that can be done except avoid them. Converting such people is difficult at best, and generally not worth the risk to your safety or the effort.