Dang, I am usually not left in the dust in these sorts of discussions, but I've never heard the term 'demisexual' before.
Personally, I don't run into much trouble explaining my orientation because I'm straight. But for other parts of life where this comes up ... like if someone asks my job, sometimes I feel comfortable giving the whole story. Sometimes I don't (whether because I'm pressed for time, or because the person doesn't seem genuinely interested), and then I'll just say "engineering.&quo t;
It's not lying, it skips a conversation I don't feel like having, so both of those are pros to 'dumbing it down'---but I do miss an opportunity to evangelize about how awesome my (small, sometimes misunderstood) field is. I guess I assume that telling people you're bisexual when you're really pansexual is similar---saves you hassle, but you miss an opportunity to educate someone.
But at the end of the day---it's not your job to educate everyone you meet on the street. If it doesn't give you a headache, don't dumb it down, and give someone a new word for their vocabulary. But if you don't feel like having the conversation, go ahead and just say 'bisexual.' Or even 'it's private.' You're under no obligation to share that info with anyone, really.