You don't need the weight of a rider on a horses back to get them seeking the rein. Have a look at the horses in the Spanish riding school. They are all worked in side reins without a rider and look at the astonishing levels of collection they achieve.
Of course you will teach a horse to lean on the rein or back off it in side reins if you don't know how to use them efficiently. Hence my constant emphasis on 'experienced' rider/handlers knowing how to use the reins, what they want to achieve from their use and how exactly they work.
People seem to be under the impression side reins are their to crank the head in and that's it. That is an extremely misguided and uneducated idea.
Side reins should be adjusted to a length that has a small amount of slack in them when the horse is working just in front of the vertical (if the horse is at a level where they know how to work on the bit and in the early stages of collection, for young/green horses of course you will let the reins out longer to get the neck stretch). From that, you MUST drive the hind legs, expect them to really motor along and take their weight on the hind legs. Exactly the same principals apply on the lunge as they do under saddle. You want the hind legs to be driven forward, transferring energy over the back which in turns begins to soften and 'swing', and up over the neck and through to the bridle, thus taking a steady, but relaxed contact with the bit. If you don't drive the hind legs, then of course the horse is going to fall on the forehand and lean on the reins. If he is working powerfully from driving hind legs, his forehand will soften and he will not learn to rely on the reins for support.
AGain I refer to my initial post. How (if you are a dressage/flat rider who knows about outside rein connection, straightness, throughness etc. ) do you lunge your horse and maintain a connection to the bit, a straight body, swinging back with not outside rein contact? Unless the horse is highly trained under saddle and works naturally straight, or he is a saint, the energy you create in the hind legs will flow straight out through the outside, or inside shoulder, without an outside rein to contain the flow of energy, thus creating a crooked horse who has now found a good evasion to use under saddle.