I'm with Glenn - - what do you mean?

By "external or internal", I assume you mean somebody else's reasons or your own reasons. It can mean either. It could be external because others do not approve, or internal because you now realize that you are clearly are not meant for one another so it is best to part ways.

The phrase "we won't be able to date" generally sounds like an external statement. It suggests a situation or circumstance that is preventing the possibility of dating, rather than expressing a personal internal feeling or decision. It implies external factors or constraints that are influencing the ability to pursue a romantic relationship.

The phrase would suggest that you could not date due to outside or external influences, i.e. something other than the feelings/wishes of either person was preventing the relationship.