I think the difference is the perceived energy barrier if one wanted to fool around on their partner if they a bi vs het.
A bi guy could, hypothetically, find a guy on grindr pretty much on a whim if he wanted to. This is a much lower barrier than straight guys face unless they seek a sex worker. If you are a woman in the early stages of dating someone, where you don’t know yet how much you can trust a guy, if a guy tells you he is bi that can come off as a higher cheating risk than a straight guy.
Straight guys dating a bi-girl don’t have a similar perceived risk increase. Early in the relationship, guys may not even see the potential of a bi-girl hooking up with a girl as ‘cheating’, vs a bonus for his enjoyment. But also - finding a new girl to date is considered harder than finding an interested guy. So the ‘cheating’ risk doesn’t feel that much higher for guys dating a bi-girl compared to a straight girl; he may feel like he is still mostly competing against other guys.
Is this fair or even realistic? No, this is based on perceived stereotypes rather than the behaviors and character of individuals.
But this plays out at a stage of dating where people don’t know each other well yet and are relying on heuristics.
To be fair, this kind of kinetics-as-pevention works.
I use this strategy to prevent eating too many snacks – I do not keep them at home. I can’t have them if they are not here. If 90% of the time I would “choose” not to have chips, not-having-them stops me the other 10%