I completely and utterly disagree with your first malicious, ignorant, bigoted remark that most demons are unappealing.
You are unappealing!
However, that being said, like all demons, the appearance of a ‘bai is shaped by the first summoner and the demon. And the first summoner is in particular looking for a sex demon. I can’t say much more on this without going into stuff from Book 2.
Thus they all tend to be rather good looking by Astlanian standards. They are also almost always well endowed. They often have accessories/appendages/body structure that makes them very proficient at giving pleasure (prehensile tongues, inflatable/extensible/dextrous digits (fingers/toes) etc.
They also all radiate a charm/lust/arousal/attraction aura, causing lesser beings to become quickly aroused and in love.
The more powerful demons have illusionary capabilities that make their target see their most desired lover. And different people looking at the same time, may see the demon differently.