It sounds okay to meCan I say Je ne porte pas un jean even if I don't go on to say Je porte une jupe ?Does using the indefinite article suggest that I am wearing something other than jeans?
No, because the word jeans is specific enough. On the other hand, Je ne porte pas de pantalon is much more likely to be understood as I'm not wearing anything over my underpants / I'm not wearing pants.Now that I think of it, does Je ne porte pas de jean mean that I am wearing nothing over my culotte?
Language is not an "exact science", though. I can definitely imagine a situation where a girl would be talking to her boyfriend on the phone, saying "Je ne porte pas de pantalon". Her friend could ask "Tu es nue ?!" and then she would cool him down saying "Non. Je porte une jupe". You can perfectly play with this ambiguity.
Note that Je ne porte pas de jean might be understood as a habit: I (usually) don't wear jeans.
If a friend of mine made a compliment on my "jeans" (which actually aren't jeans), and I wanted to correct him (however unfriendly it might sound!), I would say (in order of preference):
.....1. Ce n'est pas un jean que je porte.
.....2. Je ne porte pas de jean, là (= "maintenant").
.....3. Je ne porte pas un jean.