Students (kids and adults) all over the world are learning upright bass on a big slab of unlined ebony and are not experiencing any more stress than the generations that have done the same thing before. They aren't experiencing any more stress than a fretless bass player experiences. Not with the fretting hand anyway. Fretless bassists ought to be glad they only have to learn the easy technique - left hand placement/intonation. They don't have to learn the hard part of double bass - bowing/arco skills. So... Why should bass guitarists sweat over fretted/fretless or lined/unlined? They have it easy by comparison. I play both professionally, so I'm comfortable with the comparison.
To directly answer the OP's question, there is no reason to not consider a fretless for learning. Fretted basses are...well... fretted basses. It might be easier to learn on a fretted, but not necessarily better. If your intention is to play fretless bass, then simply play fretless bass. As you learn bass basics, you'll be learning to control intonation. Just like upright bass players do.
Lined or unlined fretless? Some double bass students (children) often have their basses marked when they first start, but those markings are removed soon, or they grow into larger basses sans markings. I don't think an adult student needs those markers, although they do act as a guide/target until technique and muscle memory (from lots of practice) take over. There is no harm in starting with a lined fretless, but at some point, unless you are a casual player/hobbyist, the lines should become completely unnecessary. They should be a learning tool, not a crutch.
Learning fretless bass does not require learning on a fretted bass. A good teacher, however, goes a long way toward success no matter what bass you start on.
My $.02
Bill
