I played a P sitting down 5 days a week for 5 years. Yea it was a 70s P, not the lightest, but not uber heavy either.Very interesting!!! I got an email from Fender, answering my question about neck dive. This is a part of it:
A bit of neck dive is common on any P-bass, across all series. As you have noted, these guitars are more happy when used with a strap.
The Jazz basses are bit more 'balanced' due to lower weight on neck and offset body shape, this can potentially facilitate a more comfortable experience when played sitting down without a strap.
Mustang basses tend to be significantly lighter and with a shorter 30" scale, this makes the whole balance aspect less noticeable overall.
No issues. I was a teenager also, no problems. Or standing when needed, no issues.
So just saying... I think neck dive is a relative term.